AMNA NAWAZ: President Biden has arrived in Poland tonight, after making an unannounced visit to Kyiv just days ahead of the one-year anniversary of Russia's full-scale invasion of Ukraine.

The administration says it informed Moscow before Biden arrived.

This marks the first visit by a U.S. president to Ukraine in 14 years.

With the support of the Pulitzer Center, Nick Schifrin reports from Kyiv.

NICK SCHIFRIN: At the end of a 5 000-mile journey, the commander in chief visited a capital at war to provide an embattled country a show of solidarity.

On the right, the leader who one year ago refused a U.S. offer to evacuate wearing trademark olive drab, on the left, the leader who became Ukraine's most important supporter wearing a tie striped with Ukraine's national colors.

JOE BIDEN, President of the United States: I'm here to show her unwavering support for the nation's independence, your sovereignty and territorial integrity.

NICK SCHIFRIN: The administration today called President Biden's visit unprecedented in modern times, to a war zone without a significant U.S. military presence.

Russia launched its full-scale invasion 361 days ago.

President Biden remembered it seemed dire.

JOE BIDEN: One year ago, the world was literally at the time bracing for the fall Kyiv, perhaps even the end of Ukraine.

You know, one year later, Kyiv stands and Ukraine stands.

Democracy stands.

The Americans stand with you and the world stands with you.

Kyiv has captured a part of my heart, I must say.

VOLODYMYR ZELENSKYY, Ukrainian President (through translator): We must ensure that 2023 becomes the year of victory, with the liberation of Ukrainian land from Russian occupation, with solid guarantees of long-term security of our country, Europe and the entire world.

NICK SCHIFRIN: But while the U.S. today announced another $500 million package of weapons transfers, it did not include long-range weapons that Zelenskyy once again requested.

VOLODYMYR ZELENSKYY (through translator): Such an important package is an unmistakable signal that Russian revenge attempts would have no chance.

NICK SCHIFRIN: The U.S. informed Moscow of Biden's travel, but, still, Moscow scrambled jets, triggering Ukraine's air raid sirens.

It came during a visit to St. Michael's Cathedral, which became sanctuary during the 2014 protests known as the Revolution of Dignity.

On this date every year, Ukraine remembers the more than 100 protesters killed nine years ago, helping evict a pro-Russian president.

Today, Biden and Zelenskyy remembered the more than 4,500 soldiers killed since then fighting Russia.

JOE BIDEN: Thank you for everything you have done.

NICK SCHIFRIN: Inside, Zelenskyy and Biden met with Orthodox Church of Ukraine leaders who've helped forge an independent religious identity distinct from Russia.

And even in a country at war, there was time for a quiet moment.

This was the scene outside during that portion of the visit.

This is as close as we could get, about 600 or 700 feet away.

There was unprecedented security here in the center of Kyiv.

This is usually a bustling street.

The city center was locked down and mobile and Internet service cut off around Biden.

The presidential convoy drove quickly through empty streets.

Biden is no stranger to Kyiv.

He visited some of these same sites back in 2014 after Russia's initial invasion, but his trip today, right before a sad milestone, was the trip Zelenskyy and his team wanted most.

JOE BIDEN: Freedom is priceless.

It's worth fighting for as long as it takes.

And that's how long we're going to be with you, Mr. President, for as long as it takes.

AMNA NAWAZ: And Nick joins me now from Kyiv.

Nick, it's good to see you.

As you know, it's no small task to move the president of the United States thousands of miles into a war zone in secret.

What do we know about the planning and the execution of this historic trip?

NICK SCHIFRIN: It was a trip that was kept secret from all but the senior officials in each agency who was actually planning the trip.

And the pool reporters who accompanied Biden along the way report that President Biden left Washington, D.C., at 4:00 a.m. on Sunday in a smaller-than-normal plane that had been kept darkened and off to the side.

He landed in Eastern Poland,drove to the border, and then took a nine-and-a-half-hour train to Kyiv.

He had to take a train, of course, because the airspace here has been closed since last February.

And that is the train he took, arriving back to Poland from Ukraine tonight.

It's the same route that we all take, that all previous heads of state in government have taken for the last year.

But this, Amna, is definitely not the same train the rest of us take.

Take a look at this photo.

President Biden, National Security Adviser Jake Sullivan even had an office.

U.S. officials say that it took them months of meticulous planning to get Biden safely here.

AMNA NAWAZ: Nick, this trip comes at a critical time.

We're nearing the one-year mark in this war.

What do we know about what the discussions unfolded -- how they unfolded between President Biden and President Zelenskyy?

What did they talk about?

NICK SCHIFRIN: It's a critical moment not only because of that anniversary you just mentioned, Amna, but because of the fighting has already increased across Ukraine's east and is expected to intensify.

Russian forces, according to U.S. and Ukrainian officials we speak to, have already launched an offensive, with the goal of capturing all of Donetsk province.

The fighting is focused around Bakhmut, but spreads north and south from there across 100 miles.

And the presidents also discussed Ukraine's plans for a counteroffensive expected in the coming weeks in the south of the country.

And U.S. officials I speak to about that counteroffensive are split on the chances of success.

But the administration officials who are helping plan the next few weeks in Ukraine believe that new training and new armored vehicles coming to Ukraine should give Kyiv the ability to create at least local advantages along the front line in the south, where Russian troops have really been digging in for many, many months.

But where U.S. and Ukrainian officials do not agree, Amna, those long-range weapons that we reported in the story that Zelenskyy once again asked for and that, as of now, the U.S. is still refusing to provide.

AMNA NAWAZ: Nick in the meantime, in the last couple of days, we have heard from Vice President Harris, from Secretary Blinken public calls warning China against supplying lethal aid to Russia.

What's behind that?

What should we know?

NICK SCHIFRIN: Yes, that warning came from Antony Blinken three separate times this weekend in public and also during a meeting that he had with China's top diplomat, Wang Yi, at the Munich Security Conference.

U.S. officials are concerned because, according to congressional officials I speak to, Chinese companies have begun to send dual-use technology, including surveillance items, to the Wagner paramilitary group that is leading the fighting in Bakhmut and to the Ministry of Defense.

A senior U.S. official told me that Beijing was likely -- quote -- "looking away" from these sales.

And so what U.S. officials are trying to do is to make sure that this dual-use technology isn't passed through more official channels, as in Beijing would actually authorize the dual-use technology from being sent, but also that step that you mentioned, Amna, that Beijing doesn't cross the threshold and send lethal aid, send weapons to Russia.

Obviously, in the short term, the U.S. doesn't want to see anyone help Russia in these crucial weeks and months in Ukraine.

But, long term, the U.S. believes that its export controls on Moscow will cripple the Russian military in the next few years.

And one of the few avenues that Russia could turn to bail it out and still field a modern military is Beijing.

And that is what the U.S. does not want to see happen.

AMNA NAWAZ: Nick Schifrin reporting tonight from Kyiv.

Nick, good to see you.

Thank you.

NICK SCHIFRIN: Thanks, Amna.