US troops and tanks have begun arriving in Poland, part of one of the largest deployments of US forces in Europe since the Cold War.
The deployment will eventually involve more than 3,000 soldiers.
The Pentagon operation has sparked immediate anger from Russia, with the Kremlin describing it as a "threat" on its "doorstep."
The Atlantic Resolve mission will see an armoured brigade with more than 3,000 Amercian soldiers and heavy equipment deployed in Poland and nearby NATO partners Estonia, Latvia, Lithuania, Romania, Bulgaria and Hungary on a rotational basis.
The outgoing Obama administration ordered the deployment in 2014 to reassure eastern allies after Russia's actions in Ukraine.
However the operation comes a week ahead of the inauguration of US President-elect Donald Trump, who has suggested his Republican administration will seek to ease tensions with Russia.
A contingent of US troops entered Poland at the Olszyna border crossing with Germany.
Heavy equipment, including 87 Abrams tanks and over 500 personnel carriers including military-equipped Humvees were to follow.
"This operation threatens our interests and our security," Kremlin spokesman Dmitry Peskov said.
"This is even more pronounced when a third party (the United States) reinforces its military presence on our doorstep in Europe," he added.
Russian deputy foreign minister Alexei Mechkov described the deployment as a "factor destabilising European security."
Last summer, NATO leaders also endorsed plans to rotate troops into Poland and the three Baltic states to reassure them they would not be left in the lurch if Russia was tempted to repeat its 2014 Ukraine intervention.
A separate US-led battalion working within the NATO framework will be stationed near Poland's north-east border with Russia's Kaliningrad exclave.
Moscow has stepped up its presence in the Baltic Sea area over the last two years.
Its jets regularly violate the airspace of smaller ex-Soviet NATO allies like Estonia and in April they even buzzed a US naval destroyer.
Late last year, Poland criticised Russia's deployment of nuclear-capable Iskander missiles into its Kaliningrad outpost that borders NATO members Poland and Lithuania.