Could a NATO build-up in Europe reignite the Cold War?

GWEN IFILL: It's been a generation since the Cold War ended, a standoff that dominated much of the 20th century.

But now echoes of that conflict are sounding again, as the U.S. and its allies encounter a resurgent Russia.

Chief foreign affairs correspondent Margaret Warner reports.

MARGARET WARNER: NATO is planning its biggest buildup in Eastern Europe since the Cold War, all to deter a newly assertive Russia.

ASHTON CARTER, Secretary of Defense: We haven't had to worry about this for 25 years. And while I wish it were otherwise, now we do.

MARGARET WARNER: Last week, Defense Secretary Ash Carter proposed to quadruple U.S. spending on its — quote — "European reassurance initiative" to $3.4 billion.

ASHTON CARTER: That will fund a lot of things: more rotational U.S. forces in Europe, more training and exercising with our allies, more prepositioned war-fighting gear and infrastructure improvements to support all this.

MARGARET WARNER: This week in Brussels, NATO's defense ministers will discuss setting up outposts along its eastern front to do just that. But is this wise or necessary?

Evelyn Farkas just left her post as assistant secretary of defense for Russia, Ukraine and Eurasia.

EVELYN FARKAS, Former Deputy Assistant Secretary of Defense: This is something that we absolutely need to do. It is a sign of our resolve. It's a sign that the United States is there with our NATO allies, that we're there also to send a signal to Russia. I mean, it's clearly a deterrent effort.

MARGARET WARNER: But Jack Matlock, ambassador to the Soviet Union from 1987 to 1991, warns it will simply provoke Russian leader Vladimir Putin into further aggressive moves.

JACK MATLOCK, Former U.S. Ambassador to the Soviet Union: I think it's a bad idea. I think it's not necessary. I think it's going to lead to even more confrontation with Russia. And, probably, simply they will move more of their military equipment to the border.

MARGARET WARNER: Estonia's ambassador to the U.S., Eerik Marmei, echoes all three Baltic nations in insisting they need the protection.

EERIK MARMEI, Ambassador, Estonia: It a very clear sign of the U.S. commitment to enhance the deterrence in Europe, and especially in the eastern part of Europe in the coming years.

MARGARET WARNER: Why do you need more?

EERIK MARMEI: What we have seen in recent years is the change of security environment in Europe. We have seen that Russia has violated main principles of international agreements, law.

MARGARET WARNER: He's referring to Russia's 2008 invasion of the former Republic of Georgia, capturing two pro-Russian territories. It triggered the deepest rift between Russia and the West since the fall of the Soviet Union.

Tensions exploded anew in 2014, when Russia invaded and annexed the Crimean Peninsula of the former Soviet Republic of Ukraine.

PRESIDENT VLADIMIR PUTIN, Russia (through interpreter): This strategic territory should be under strong and stable sovereignty, which, in fact, could be today only Russian.

MARGARET WARNER: The Kremlin then provoked war in mainland Ukraine's East between pro-Russian separatists and government forces. With more than 9,000 dead, the conflict continues.

Also sounding alarms in the West, Russia's recent multibillion-dollar military buildup, with large-scale ground exercises, overflights of neighbors and a spike in submarine activity.

NATO's commander in Europe, U.S. General Philip Breedlove, has issued repeated warnings.

GEN. PHILIP BREEDLOVE, Supreme Allied Commander Europe, NATO: Russia is blatantly attempting to change the rules and principles that have been the foundation of European security for decades.

MARGARET WARNER: But how far do Putin's ambitions extend? It's one thing to reassert influence in former Soviet republics, like Ukraine, Georgia and Moldova. But would he really move against once independent countries the Soviets took over during World War II who are now free members of the NATO alliance?

NARRATOR: This union of 12 nations became known as the North Atlantic Treaty Organization, or, more simply, NATO.

MARGARET WARNER: NATO was formed in April 1949 with 12 members to defend themselves from the Soviet Union based on the principle of an attack on one is an attack on all.

But after the Berlin Wall fell in 1989 and the Soviet Union collapsed in 1991, many former Soviet satellite states and Republicans sought and won the protection of NATO's security umbrella. Most galling to the Russians was NATO's 2004 incorporation of three tiny Baltic states, former Soviet Republicans where many ethnic Russians still live, Lithuania, Latvia, and Estonia.

PRESIDENT BARACK OBAMA: We will defend our NATO allies, and that means every ally.

MARGARET WARNER: In September 2014, after the Ukraine crisis erupted, President Obama went to Estonia to reassure the Baltics and to warn the Russians.

PRESIDENT BARACK OBAMA: So, if, in such a moment, you ever ask again, who will come to help, you will know the answer, the NATO alliance.

MARGARET WARNER: But is Russia actually a threat to its Baltic neighbors?

EVELYN FARKAS: Absolutely. Absolutely. You know, all of the countries on Russia's periphery have been either invaded by Russia and occupied. The Russian government, this Kremlin has a foreign policy that essentially asserts their right to have political and economic control over their periphery.

MARGARET WARNER: What do you assess is Putin's intention vis-a-vis these countries?

EVELYN FARKAS: Regionally, he wants to maintain this political and economic control. It doesn't mean he has to invade every country. He can also try to undermine the countries through other means.

JACK MATLOCK: I don't think there's any possibility of Russia making an incursion in the Baltic states.

MARGARET WARNER: Former Ambassador Matlock says the Russians are simply responding to what they see as NATO squeezing them by expanding right up to the Russian border.

JACK MATLOCK: In their eyes, they have not committed aggression, that they are responding to aggression from the West, and particularly from the United States, which they accuse, I think unfairly, but sincerely, of trying to encircle them with military bases.

MARGARET WARNER: Western leaders concede they have little insight into what drives Putin or what he intends. But mutual hostility is growing and, Matlock warns, dangerous.

JACK MATLOCK: There has been created a almost Cold War atmosphere of hostility. In that atmosphere, you are going to make it very difficult to cooperate on bigger issues.

MARGARET WARNER: For example, he says, battling Islamic State terrorism. And then there's the nuclear threat between two superpowers.

JACK MATLOCK: It would be a catastrophe to get into a war with Russia. And it seems to me that you need to deal with a certain prudence.

MARGARET WARNER: Do you think that we are sliding into some kind of new Cold War in our relationship with Russia? Should it make us tread more carefully?

EVELYN FARKAS: Not necessarily. I mean, we should always tread carefully, but if by that you mean that we shouldn't signal resolve to Russia, I would say no. We have to signal resolve to Russia.

MARGARET WARNER: The next signal of that resolve should come in Brussels later this week.

In Washington, I'm Margaret Warner for the PBS NewsHour.