The United States is sending troops by the thousand and dozens of tanks to Eastern Europe in the largest deployment of troops and equipment since the fall of the Soviet Union.
According to the BBC:
The equipment and 3,500 US troops are to be deployed along Nato's eastern frontier. The deployment aims to allay worries of potential Russian aggression in eastern Europe.
However, some fear the large number will exacerbate tensions with Moscow.
In 2014, Russia invaded and annexed Crimea, a Black Sea peninsula that had been under Ukrainian administration up until that point. North Atlantic Treaty Organization (NATO) allies at Russia's western border expressed concern in October 2016, when that country sent nuclear-capable Iskander missiles to Kaliningrad, a Russian province sandwiched between Lithuania and Poland on the Baltic Sea:
The troops are part of President Barack Obama's response to reassure Nato allies concerned about a more aggressive Russia.
It is the largest US military reinforcement of Europe in decades.
President Vladimir Putin's spokesman Dmitry Peskov told the BBC that the move "threatens our interests and our security".
"It's a third country that is building up its military presence on our borders in Europe," he said. "It isn't even a European country."
The deployment comes just days before the 20 January 2017 inauguration of President-elect Donald Trump. Trump's friendly overtures toward Russian President Vladimir Putin have raised questions in Eastern Europe about security under the new American administration.