GWEN IFILL: From Brussels to Havana to the 2016 campaign trail, terror, history, and political feuding pretty much sums up the week, tonight on Washington Week.

BRUSSELS RESIDENT: (From video.)

It's a little bit kind of war - the feeling of war.

BRUSSELS RESIDENT: (From video.)

We all know that we're not safe anywhere.

It can happen anywhere and at any moment.

GWEN IFILL: More bloodshed, more terror, more worry about whether ISIS can be stopped.

GENERAL JOSEPH DUNFORD: (From video.)

The momentum is in our favor, but by no means would I say that we're about to break the back of ISIL or that the fight is over.

GWEN IFILL: The Brussels attacks jolt the world's attention back to a growing threat.

SECRETARY OF STATE JOHN KERRY: (From video.)

Now we declare: Je suis Bruxellois.

GWEN IFILL: Air Force One touches down in Cuba, bringing an American president to the island nation for the first time in 88 years.

PRESIDENT BARACK OBAMA: (From video.)

I have come here to bury the last remnant of the Cold War in the Americas.

(Cheers, applause.)

I have come here to extend the hand of friendship to the Cuban people.

GWEN IFILL: But as this immediately iconic picture showed, the hand of friendship only goes so far.

At home, campaign 2016 devolves into a bitter fight between GOP frontrunner Donald Trump and Ted Cruz.

SENATOR TED CRUZ (R-TX): (From video.)

If your car is broken down, do you want to bring a guy over to stand in the driveway and yell and scream and curse at your car - (laughter) - or do you actually want someone to lift the hood and fix the darn thing?

GWEN IFILL: While lagging John Kasich soldiers on.

OHIO GOVERNOR JOHN KASICH (R): (From video.)

Let me tell you, I drop out, Donald Trump is absolutely going to be the nominee.

GWEN IFILL: Covering the week, Mark Mazzetti, national security correspondent for The New York Times; Nancy Youssef, senior national security correspondent for The Daily Beast; Tom Gjelten, religion correspondent for NPR; and Robert Costa, national political reporter for The Washington Post.

ANNOUNCER: Award-winning reporting and analysis.

Covering history as it happens.

Live from our nation's capital, this is Washington Week with Gwen Ifill.

Once again, live from Washington, moderator Gwen Ifill.

GWEN IFILL: Good evening.

The random nature of terror attacks makes them all the more terrifying.

So it was this week in Brussels, where an apparent ISIS cell went after the kind of soft targets - the airport, the subway - that ordinary citizens frequent.

Dozens died, hundreds more were injured, and world leaders were left struggling once again with questions about how to cut off the roots and the branches of ISIS.

PRESIDENT BARACK OBAMA: (From video.)

This is difficult work.

It's not because we don't have the best and the brightest working on it.

It's not because we are not taking the threat seriously.

It is because it's challenging to find, identify very small groups of people who are willing to die themselves and can walk into a crowd and detonate a bomb.

GWEN IFILL: That takes intelligence and military might.

But what happens when terrorists take advantage of porous borders to make Europe their prime target, Mark?

MARK MAZZETTI: What happens is you get Paris in November and you get Brussels this week.

And in many ways, the two attacks are very connected, and one is an extension of the other.

There's a number of things going on that are really problematic in Europe.

Not only do you have an influx of people coming into Europe, and no real clear sense of who's coming in.

But also around Europe there's very little monitoring about who's going across porous borders, as you said, and intelligence services that are really overwhelmed.

GWEN IFILL: And fractured.

MARK MAZZETTI: And fractured.

And so you - what investigators have found after the Paris attacks in November was that ISIS had established a pretty big infrastructure in the city, and also in Europe, to carry out these kinds of attacks.

And what we found this week was that a lot of the attackers, the people involved in the Brussels attacks, were the same people who carried out Paris.

So really - it's really two attacks that are very, very connected.

GWEN IFILL: Well, and the question immediately turns to, so what do you do about it?

So here in the United States today at the Pentagon they actually said they were making some progress against ISIS.

NANCY YOUSSEF: That's right.

They announced that they had killed the minister of finance - the ISIS minister of finance, Abdul Rahman Mustafa al-Qaduli.

And this was a significant deal because one of the ways that ISIS is able to expand its grip outside of the Islamic State territories is through financing European nationals who look to launch attacks.

It was an interesting attack as well, because it's the latest in a string of attacks that the United States has been able to conduct against ISIS finances, starting with the raid in May of last year against Abu Sayyaf and followed by the strike on three buildings that held upwards of $750 million for ISIS.

And so we're starting to see increased success at going after big-name members of the Islamic State.

GWEN IFILL: The administration has come under some criticism for not having a strategy, not just a global strategy but a strategy to have boots on the ground, so to speak.

But is this a geographic fight anymore, or is this more ideological?

Because they seem to have tentacles everywhere.

NANCY YOUSSEF: Oh, I was just going to say, well, the challenge is, on one hand you have the Islamic State losing territory in Iraq and Syria, and at the same time has been able to do increasingly sophisticated attacks in Europe.

So it really raises a question: Does ISIS need territory to pose a threat to Europe?

And I think the events of this week has really raised that question.

It seems that essentially that it's a little bit of both, that the territory allows them to expand their grip on Europe.

And what we're, I think, really seeing is that even when ISIS endures strikes in its territory, it has attacks in the works for upwards of a year or two years.

And so they can be weaker now in Iraq and Syria, and yet still be effective, at least in the short term, in Europe.

MARK MAZZETTI: And I think we're going to see this sort of punch and counterpunch aspect of this, of them attacking in the West in places where they can, like in Western Europe, the United States and coalition forces continuing to hit them in Syria.

And you know, there was this debate before Paris, in the months before Paris, is, was ISIS like al-Qaida?

Were they going to do the types of attacks?

Well, the debate - whether they were or not, they are now, in the sense of the type of attacks that they're going to carry out.

So I think we should be expecting them.

TOM GJELTEN: Now, almost all the attackers in Paris were killed.

A bunch of the attackers in this case were killed, including the guy allegedly who made the bombs.

I mean, how much of the network, you know, is still out there?

Because a lot of the guys have been arrested, a lot of guys have been killed.

What's left there?

Do we know?

MARK MAZZETTI: Nobody that I think either of us have talked to think that they've wrapped this whole thing up.

They're clearly still looking for others.

They, you know, don't know what they don't know, in the sense of the extent of the network.

And this may just be one cell that is focused on France and Belgium.

There may be other cells.

Again, there was real surprise after Paris, once they dug into the attack, just how extensive the support network was.

And, you know, French officials I've spoken to talk about the number of foreign fighters who were able to go back and forth to Syria.

And so there is real fear that this is just something we're going to see semi-regularly.

ROBERT COSTA: Why was Brussels a target?

Not just in terms of the vulnerability of the city in terms of its intelligence, but was there any symbolism to picking Brussels?

NANCY YOUSSEF: Well, I think one of the areas in which it was particularly susceptible is that it has a migrant community that feels isolated from the rest of the country.

You have third-generation Moroccans, for example, who still identify themselves as Moroccan-Belgians, that they're not - they don't feel Belgian and they don't feel that they're a part of the community.

And so you've had this going on for quite a period of time, and so it's made Brussels, and Belgium writ large, susceptible to an Islamic - radical Islamic network coming into the country.

And I think that's been a challenge that Belgium in particular has had in terms of combatting the Islamic State threat.

You can't have people living in a country and not feeling a part of it.

They're less likely to go to the authorities when they see something suspicious.

And you have now a huge enclave where plots can be - where there can plots and where people can hide.

The apartments, for example, in which the suspects were found last week - Salah Abdesalam and then the suspects in this attack - were in neighborhoods and able to hide for several months.

GWEN IFILL: One of the most disturbing features of this whole investigation is that there doesn't seem to have been that much collaboration between the authorities - you mentioned the authorities in Brussels and in Belgium - and anybody anyplace else in the world.

The president of Turkey said, hey, I warned these guys.

He deported them to the Netherlands, but I warned every one of these guys were bad guys.

Turned out maybe some of them were on the U.S. no-fly list.

Why is it that this wasn't - not why didn't they see it coming, but why wasn't it better coordinated?

MARK MAZZETTI: It's a mess right now, actually, in terms of the intelligence sharing, in terms of the structure of the spy services around Europe.

I mean, we learned the hard way 15 years ago how our intelligence services didn't talk to each other.

The FBI and the CIA weren't talking to each other.

That was one of the reasons why the September 11th attacks happened.

There's been a lot of progress in the U.S. since then.

Europe is in that sort of state before the September 11th attacks where they don't talk to each other, there's a lot of historic rivalries, they've fought wars against each other.

And even - not only country-to-country, but within countries.

In France, certain services won't talk to each other.

So this is a real problem.

TOM GJELTEN: Nancy, you raised an interesting question about whether the losses in Syria are all that significant.

But isn't it true that one of the recruiting pitches of ISIS has been this caliphate, they control territory, they have this mini-state?

So, you know, how are those two considerations balanced?

NANCY YOUSSEF: Well, on one hand that state has been appealing, but on the other hand there's the inspiration of waging war on behalf of Islam throughout the world.

And so it's not clear that the state has required so much as the idea that Islam is under attack, and it is your job as a disaffected Muslim in Europe to fight back.

But it's an interesting dilemma because the United States is saying we're having an effect, ISIS is weaker, and yet - and yet the attacks get more and more sophisticated.

And not only that, you've seen ISIS that is able to respond within days to what seem to be major setbacks for the group.

Salah Abdesalam was arrested four days before Belgium.

And we've seen this pattern where ISIS is able to respond very quickly to setbacks.

They're able to control the narrative as say that what seems like a setback to you is us strategically and smartly evolving and adapting to these setbacks.

MARK MAZZETTI: And they may have even been able to - after that arrest last Friday they may have thought, oh, we need to accelerate those attacks.

So they just moved quickly and were able to - and they've had successful bomb makers carry out this attack, send out a video the next day, proclaim great victory.

So it's a fairly agile organization.

GWEN IFILL: Briefly, what else should we - San Bernardino, which was ISIS-inspired, but not necessarily ISIS-directed, should we be domestically more worried about that type of attack than the kinds of networks we're seeing in Europe?

NANCY YOUSSEF: Well, the challenge is it's both, in a way.

There's not the kind of network in the United States as there is in Europe.

And it's clear that it's well-established in Europe.

But the idea that you don't know how much of it is inspired and how much of it is being funded by ISIS I think makes it very hard to make any safe predictions about what will motivate the next attack.

GWEN IFILL: OK.

Thank you both.

On to Cuba, where the president crossed a historic item off his bucket list by navigating a delicate tightrope with a communist nation that he said represents a new beginning.

PRESIDENT BARACK OBAMA: (From video.)

There's no secret that our governments disagree on many of these issues.

I've had frank conversations with President Castro.

But here's what the Cuban people need to understand: I welcome this open debate and dialogue.

It's good.

It's heathy.

I'm not afraid of it.

GWEN IFILL: There were predictable criticisms.

House Speaker Paul Ryan said the trip, quote, "legitimizes a tyrannical dictatorship."

But did the president's careful words of conciliation resonate in Havana, Tom?

TOM GJELTEN: Well, Gwen, I can tell you the vast, vast majority of the Cuban people have been waiting for this moment for years and years and years.

And the people that we had there say that the Cubans were glued to their television sets, they were cheering him.

I mean, this is just stunning.

This is something that's been unimaginable for many years.

I think, on the other hand, the leadership had to be a little bit disoriented by this whole experience because, you know, the Cuban regime has had this almost existential need to have the United States as its - GWEN IFILL: - as its enemy.

TOM GJELTEN: - as its enemy.

And you know, you had Raul sitting up there and watching this and sort of politely clapping.

But I was interested in the official commentary afterwards in the official government press was very negative.

You had commentators saying, the enemy is our enemy, the past cannot be forgotten.

So clearly there, you know, is some discomfort there.

You know, to me, this was something like the foreign policy equivalent of pulling out of Vietnam.

You fight a war for 50 years and at some point you just say, you know what?

We lost.

It didn't work.

GWEN IFILL: But it seems like - there's also a lot of distinction to be drawn by the people who run the country and the Raul Castros of the world and his lieutenants, and the people of Cuba, many of whom are Afro-Cuban and took a look at President Obama and said, yeah, I get this.

TOM GJELTEN: Well, that is - about two-thirds of the population of Cuba at this point - so many, you know, of the Cuban exiles are white that about two-thirds of the remaining population is either Afro-Cuban or partly Afro-Cuban.

Not only that, you have this guy, he comes in and he says, you know, none of that stuff - I wasn't even around when this stuff happened.

You know, I was born the year of Bay of Pigs.

GWEN IFILL: Yeah, he was born the year of the Bay of Pigs.

TOM GJELTEN: So you've got his youth on - you know, he's got his youth and his Afro-Cuban background.

And this is something that definitely resonates with the Cuban people.

NANCY YOUSSEF: That's interesting, the dynamic that you talk about in terms of how people were receiving his visit.

What does that mean, then, going forward for things like trade?

TOM GJELTEN: Well, you know, it's the - here's the truth, is that after December 2014, when this announcement of an opening was made, the arrest of political dissidents has actually gone up.

It's now at a five-year high.

It's pretty clear that this is a government, it is not anxious to lose control.

And there is some worry - there is some concern that too much of this opening will actually do that.

We have had hundreds of delegations going to Cuba in the hopes of, like, ginning up business.

The number that have actually been able to negotiate deals is very small.

So I think that there's a clear reluctance on the part of the - on the Cuban government to really, like, let this play out in the fullest extent possible.

There is one change that the administration made just last week that could be hugely important.

They have decided to allow individual Americans to go to Cuba on their own, not as part of a tour.

And that really is a blow to the Cuban government because the Cuban government has been making money by providing tour guides, bus transportation, making them stay in hotels.

Now, all of a sudden, you know, individual Americans can go and stay in Airbnb places.

They don't have to hire a tour guide.

So that's a kind of a tourism that is sort of out of the government's control.

That could be significant.

MARK MAZZETTI: Tom, you've studied Cuba really closely for a number of years.

And you talk about how the number of dissidents - the crackdown on dissidents is at an all-time high.

Is that, do you think, somewhat tracked to the policy change in the U.S., the upcoming trip of President Obama?

Is there - what's the reason?

TOM GJELTEN: I think there's - I think there's two things, Mark.

I think - on the one hand, I think the government is very nervous about what this might mean and how it might erode their control.

The other thing is that I think, frankly, it sort of - it sort of has inspired Cubans to sort of be more activist.

I mean, you know, you can say that the increase in arrests is a reflection of the increased activism.

More people are actually demonstrating, you know, taking matters into their own hands.

So that could be an explanation as well.

ROBERT COSTA: What did we learn about Raul Castro from this whole experience?

TOM GJELTEN: Raul Castro is - I would say that this is not something that Fidel would have been able to handle.

Raul has always had this reputation of being more pragmatic.

I mean, the idea that Raul would go with Barack Obama to a baseball game - and of course, Barack Obama came under - came under a lot of criticism for - you know, for doing that, not only because it was right in the aftermath of Brussels, but also because the situation really hasn't improved all that much in Cuba.

But it had to also awkward for Raul.

This is not something that I can imagine Fidel would ever have done.

GWEN IFILL: No.

Watching him try to answer questions at a press conference, it was clear he had never actually been - (laughter) - he had never had a question posed to him ever before by reporters.

So he was - he was - TOM GJELTEN: He'd never been in a press conference.

GWEN IFILL: He clearly was rattled by that.

Thanks, Tom.

We end tonight where we normally begin, with the 2016 campaign.

Donald Trump came to Washington this week to invite people to get on his train.

DONALD TRUMP: (From video.)

You have a lot of people out there that you think are against me, and it's just politicians, they want to make a deal.

They want to come in and they want to be part of it.

GWEN IFILL: Hillary Clinton also began behaving as if she is already the nominee, delivering what was billed as a major foreign policy speech and taking the time to issue a pointed scolding to the GOP.

FORMER SECRETARY OF STATE HILLARY CLINTON: (From video.)

When Republican candidates like Ted Cruz call for treating American Muslims like criminals and for racially profiling predominantly Muslim neighborhoods, it's wrong, it's counterproductive, it's dangerous.

GWEN IFILL: That looks very stateswomanlike general election behavior.

Why so early?

ROBERT COSTA: Both candidates, the leading candidates in each party, see the general election approaching fast on the horizon, and they would like to pivot away from their primary battles.

On the Republican side, you see Trump, he's almost tired of this battle with Senator Cruz.

He believes he - if he doesn't reach the threshold ahead of the convention, he'll be close to it, and he relishes the idea of a fight with Secretary Clinton.

GWEN IFILL: But you say he's tired.

He didn't - doesn't sound tired, even if he's just tweeting from the sidelines.

He's still stirring the pot.

ROBERT COSTA: Politically tired.

He's ready to move on.

And Trump is at such an interesting moment, and I tracked him as he was through Washington, D.C. this past week.

He's someone who doesn't have a real core within the Republican Party.

He doesn't have a constituency, and so he's just trying to build relationships as an outsider coming in.

And to see him with Newt Gingrich and others and Senator Sessions at Jones Day law firm near the Capitol having a lunch of shrimp sandwiches and trying to just act presidential, it was - it was a turning point for him.

And it's been difficult for Trump because every time he takes a few steps forward in helping this establishment warm to him, he seems to always get drawn back into a skirmish.

GWEN IFILL: Well, that's his plan, isn't it?

Doesn't he start the skirmishes most of the time?

ROBERT COSTA: He enjoys the skirmishes, and he's someone who believes conflict can drive a candidacy.

This is not a presidential candidate who is motivated or driven by ads, or who has some kind of message beyond "Make America Great Again," is not running on a policy agenda.

It's personality.

It's conflict.

It's giving people something that's politically incorrect in an age where they feel like they can't say what they want to say.

MARK MAZZETTI: So any of the seeds that he planted this week during this trip to Washington, did he make headway?

Did it look like it was anything that was sort of building long-term relationships?

Or does it not matter?

ROBERT COSTA: I think he's made a few inroads on the right.

I mean, one of the key obstacles for Trump right now is among the conservatives who have this "never Trump" movement, and many of those people are supporting Senator Cruz.

He met with Senator Sessions and others who are close to that populist right wing of the Republican Party and said I'm with you on immigration, I have a conservative view on foreign policy to an extent in that he wants to combat ISIS - though he's not an interventionist, he's not a hawk.

And so we're seeing Trump trying to just build bridges, and it's not an easy process.

TOM GJELTEN: But you've got these endorsements like Jeb Bush and Lindsey Graham going to Cruz.

What's the significance of that?

Do they really think that Cruz can catch Trump?

Or is there some other strategic - GWEN IFILL: And what is Cruz's plan to catch Trump?

ROBERT COSTA: Cruz's plan is to have delegates accumulated.

And you see Cruz right now using his organization to his strength.

He had a big win in Utah.

Trump won Arizona.

And what you saw in Utah is what you're seeing with Cruz in places like Louisiana, where he goes to state conventions - events happening after the primary or the caucus - and then, during that event, he's able to pick up delegates or put people on the rules committee for the convention.

It still is very difficult to see how Cruz gets to 1,237, the number you need to win the nomination.

But if he can prevent Trump from getting close enough, his people feel - Cruz's people - that he can make a case at the convention.

NANCY YOUSSEF: So, given all those dynamics, where are we on the contested convention talk, as we hear about so often?

ROBERT COSTA: It seems more and more likely.

And House Speaker Paul Ryan, he's going to be moderating/chairing the convention.

He's said it's very much a possibility.

And you see Trump himself has brought in some of Ben Carson's former advisers to try to help him have a convention strategy because it's all going to be about a second ballot, when delegates become unbound.

If no one reaches the threshold, that's what happens.

GWEN IFILL: You know, I don't want to end without mentioning that today, in Portland, Oregon, there were thousands of people in an arena for Bernie Sanders.

In Washington state this weekend, he's likely to have another good time.

He's not going anywhere, either.

ROBERT COSTA: Not at all.

And I'm curious for all your thoughts on this.

When you look at the map of April, you have a lot of Rust Belt states, states in the Upper West, and these are places where Senator Sanders is strong.

And his campaign believes his is not a fading campaign.

As strong as Secretary Clinton is, he is someone who is winning over young people, and he's shown the ability to win a state like Michigan, a place that has a demolished industrial base.

And I think you're going to see Sanders continue to pick up delegates in April, and if not contest, he's going to be very competitive.

GWEN IFILL: He's certainly going to take his message all the way to the convention.

ROBERT COSTA: It's hard to see why Sanders gets out.

GWEN IFILL: Yeah, exactly.

OK. Well, thank you all very much.

This has been a fascinating week, and we didn't even talk about the spat involving the wives.

But, you know what, we can be the only people who don't do it.

You can read about it someplace else.

(Laughter.)

We are done here, but we're going to keep this conversation going online.

Who knows, maybe that's where we'll talk about it.

That's on the Washington Week Webcast Extra, where we'll tell you - also tell you what the speaker of the House had to say about this fractious campaign year.

You can watch that webcast later tonight and all week long at PBS.org/WashingtonWeek.

Keep up with daily developments with me and Judy Woodruff over at the PBS NewsHour.

And we will see you here next week on Washington Week.

And Happy Easter.

Good night.