Russia says the arrival of thousands of American troops in Poland is a threat to its national security, as the US carries out its biggest military build-up on the continent since the Cold War.
The mission, agreed at a Nato summit last year, is part of the Obama administration's attempt to ease fears among its Baltic allies following the Russian annexation of Crimea. It has taken on new significance since the election of Donald Trump, who has raised doubts about US commitments to defending eastern Europe.
The first of 3,000 soldiers and hundreds of armoured vehicles began crossing the German border into Poland yesterday, where they received a warm welcome.
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"It fulfils a dream some Poles have had since the fall of Communism – to have US troops on their soil as a deterrent against Russia," says ABC News.
But while Warsaw celebrated, Moscow issued a strongly worded statement against the deployment.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said: "We interpret this as a threat to us and as actions that endanger our interests and security."
Over the next few weeks, the combat teams will be spread across several eastern European states, including Estonia and Bulgaria.
The military operation was initially intended to begin later this month, but was brought forward at the last minute. "[It's] possibly a move by Barack Obama before he leaves office to try and lock president-elect Donald Trump into the strategy," says The Guardian.
"Trump's election raises questions as to whether this heightened US military posturing will continue," says the BBC.
The businessman, who takes office next week, has repeatedly vowed to improve relations with Russia. He has also said he would not necessarily honour the US's Nato commitment to protect fellow member states.
However, his views on Russia appear to be at odds with those of his most senior defence and intelligence nominees.
Speaking at their senate congressional hearings last night, Mike Pompeo, Trump's choice to lead the CIA, and defence secretary nominee General James Mattis both took direct aim at the Kremlin.
Mattis put Russia at the top of his "principle threat list", while Pompeo accused it of "aggressive" behaviour in Ukraine.
"While these views could reflect the nominees' desire to court favour from congressional Russia hawks, it may also set up tension between Mr Trump and his top advisers in the days and months ahead," says the BBC's Anthony Zurcher.
Nato accuses Russia of 'aggressive military posturing'
23 November 2016
Tensions between Russia and Nato have intensified following Moscow's deployment of state-of-the-art missiles in its Kaliningrad exclave on the Baltic Sea between Poland and Lithuania.
"The Bastion system fires Oniks cruise missiles, which have a range of up to 280 miles (450km)," says the BBC. Russia has already used the system in Syria.
Nato accused Moscow of "aggressive military posturing" and said the move "does not help to lower tensions or restore predictability to our relations". It called on the country to provide more information about its military activities to avoid "incidents and the risk of misunderstandings".
Russian spokesman Dmitry Peskov said the deployment was "a logical response to a hostile West", highlighting the construction of a US missile defence shield across Europe and a build-up of Nato troops near Russia's borders.
"Russia is doing everything necessary to ensure its security in the face of expansion by Nato towards its borders," he said.
The Kremlin says the aim of the US missile shield is "to neutralise Moscow's nuclear arsenal long enough for the United States to strike Russia in the event of war", reports Al Jazeera, something both Washington and Nato deny.
The increasing tension between Moscow and Nato comes as members of the EU move to strengthen military ties. On Tuesday, the European Parliament voted 369 to 255 to deepen defence coordination between member states.
'Worried' EU leaders to warn Trump over Russian sanctions
18 November
Prime Minister Theresa May and other European leaders will urge US president-elect Donald Trump to keep in place sanctions on Russia.
Downing Street said May hoped to secure an agreement to maintain the sanctions during talks with outgoing US President Barack Obama, which will address a range of issues including the threat of Islamic State and Moscow's role in the Ukraine conflict, reports the BBC.
It was Russia's intervention in eastern Ukraine that prompted Washington and Brussels to impose the sanctions in 2014.
But, during his election campaign, Trump called into question the security shield for US allies under Nato that has existed for nearly 70 years and expressed hopes for more cordial relations between Washington and Moscow.
"Trump's election upset has rattled European nerves," says the Luxemburger Wort, leaving "worried" European leaders looking to Obama for clues about the course Trump might take, "given the populist mogul's pledges to shake up the world order".
During a press conference with German Chancellor Angela Merkel yesterday, Obama urged his successor to stand up to Russia if it deviated from US "values and international norms".
He said Moscow was a military superpower with "influence around the world" but encouraged Trump to resist the temptation to "cut some deals with Russia" that damage smaller countries just because it may seem "convenient at the time".
In a bid to soothe European nerves, Obama also expressed guarded optimism that Trump could change his position once he takes over at the White House.
"There's something about the solemn responsibilities of that office... that forces you to focus, that demands seriousness," he said. "And if you're not serious about the job, then you probably won't be there very long. Because it will expose problems."
However, Lithuania believes that Russian President Vladimir Putin may test Nato in the weeks before Trump takes office. Foreign Minister Linas Linkevicius said he was "very afraid" for the Baltics and the Syrian city of Aleppo.
