PABRADE, Lithuania — On a desolate plain where Soviet tanks once prepared for a possible invasion of Western Europe, American troops are training to deter Moscow from another invasion.

Through a dense smokescreen, soldiers of the Second Cavalry Regiment advance amid intense gunfire on the imaginary positions of a NATO enemy. On their flanks are fellow NATO soldiers from Lithuania where the war game is being held — who just a quarter century ago would have been enemies, as Lithuania was one of the fifteen republics of the Soviet Union.

The U.S. and NATO are ramping up their deterrence capabilities against any Russian threat to Europe's borders —and this joint exercise is aimed squarely at Russia. NATO is drawing a red line in the three Baltic states, all of them alliance members, with a message to Moscow: "Don't cause trouble here and don't try to invade!"

American Stryker armored vehicles speed across the churned ground; later this year they will be replaced in Lithuania by Abrams main battle tanks and another rotation of American troops will arrive.

U.S. Army Capt. Russell Moore, who is leading the troops involved in the current exercise, said the war games show not just that the "the U.S. is willing to stand with all of its NATO alliance partners — It shows that we're strong, Europe is strong and there's a collective defense ready to defend against any foreign aggression."

Lieutenant Evaldas Milkintis, his Lithuanian comrade, agrees.

"We feel safer training together," he said. "We're trying to be ready to react to any threat from outside."

Lithuania has good reason to feel threatened. It was ruled by Moscow for decades until the end of the Cold War — and doesn't want to be ruled by Vladimir Putin now. That's why the small nation has watched with growing alarm as Ukraine's borders were torn up by Moscow-backed fighters.

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To its west is the Russian enclave of Kaliningrad, home to Russia's Baltic warships. And in the skies above the Baltic Sea, it watches Russian warplanes test NATO's responses.

Lithuania has become one of Russia's main adversaries because it has sent military help to the Kiev government.

Russian warplanes are constantly tweaking the West's nose, flying in formation along the borders of NATO members such as Lithuania and sometimes violating their airspace. While the flights could be merely an intimidation tactic, NATO is taking no chances because many in the Alliance believe Russia's real aim is not just to test it, but to split and destroy it.

Certainly, Lithuania doesn't trust Vladimir Putin.

"We cannot trust anything... they say, not a single word," says Lithuania's Foreign Minister Linas Linkevicius. "What's happening now it's not just about us. It's about the security of our continent."

When asked what exactly Putin wants, the minister laughs."Who knows the answer because what he's doing is irrational," Linkevicius said.

As a NATO member, Lithuania is protected by the doctrine of collective defense — in particular Article 5, which states that an attack on one member is an attack on all. Article 5 has only been invoked once in NATO's history, after the 9/11 attacks on New York and Washington.

Linkevicius suggested that even if Russia hits Lithuania with a cyberattack, Article 5 could be invoked given that it would undermine his country's strategic communications and security systems.

Still, exactly how to respond if Russia tests NATO by interfering in one of the three Baltic states is a source of debate among NATO members.

The Lithuanians are clear: there must be no mixed signals — and the country is getting ready.It has just reintroduced the military draft and has sent a booklet to every school and family advising them "what to do in case of invasion."

Russia's redrawing of the map of Europe in Ukraine has been a wake up call to the people of this young and fragile country.

NATO warplanes scramble here almost daily. People near the main NATO air base at Saiuliai used to complain about the noise from low flying warplanes, but they don't complain any more.

The leaders of the NATO alliance warn of a real and present danger from Russia —a danger greater now than at any time since end of the Cold War.

Lithuanians were the first to breakaway from Mikael Gorbachev's Soviet Union. They don't want to be back under Moscow's domination ever again.