Thousands of US soldiers began streaming into Poland on Thursday in a show of solidarity with NATO allies bordering Russia.

Operation Atlantic Resolve saw more than 3,000 US troops, backed by tanks and heavy equipment, begin their deployment in eastern Europe. The military unit will rotate through NATO's eastern flank, including Estonia, Latvia, Lithuania, Romania, Bulgaria and Hungary.

US President Barack Obama ordered the deployment in response to Russia's illegal annexation of Crimea from the Ukraine. That action, as well as the Kremlin's backing of Russian separatists in eastern Ukraine has made country's along NATO's eastern flank nervous that Russia may try a similar land-grab in their own country.

The Kremlin has responded with outrage to the military maneuvers, calling it a threat to Russian sovereignty.

"This operation threatens our interests and our security," Kremlin spokesman Dmitry Peskov said Thursday. "This is even more pronounced when a third party (the United States) reinforces its military presence on our doorstep in Europe."

Russian Deputy Foreign Minister Alexei Mechkov described the deployment as a "factor destabilizing European security."

But Michal Baranowski, director of the office of the German Marshall Fund of the United States in Warsaw, defended the military deployment.

"We have not had this level of a rotational presence since the Cold War," he said.

A new security calculus

The arrival of the US troops, along with NATO's coming deployment of four multi-national groups in Poland and the three Baltic states, "changes the security calculus on the alliance's eastern flank," Baranowski said, by creating a much-boosted deterrent.

"These forces will have a 'trip wire' function - no longer will it be possible for Russia to have a quick victory for example in the Baltic states on the cheap," Baranowski said. "It will not be able to have just a regional conflict because the NATO allies will automatically be affected - and that's a very big change."

US President-elect Donald Trump has repeatedly questioned the Western alliance's icy relations with Moscow and even the sense of the United States remaining a part fo the NATO military alliance. But during confirmation hearings on Capitol Hill on Thursday, Trump's nominee for defense secretary, retired Marine General James Mattis, expressed doubts about the prospects for cozy relations with Moscow.

Asked about the main threats to US interests, Mattis replied, "I would consider the principle threats to start with Russia."

His comments seemed likely to clinch support for his nomination, as Russia-wary lawmakers hope he will temper Trump's stated desire to partner with Moscow.

"I'm all for engagement but we also have to recognize reality in what Russia is up to," Mattis said, adding there were a "decreasing number of areas" where the US might cooperate with Moscow.

bik/sms (AFP, dpa, AP, Reuters)