The Kremlin has hit out at the biggest deployment of US troops in Europe since the end of the cold war, branding the arrival of troops and tanks in Poland as a threat to Russia's national security.
The force, intended to counter what Nato portrays as Russian aggression in eastern Europe, marks the first time that US troops will be permanently stationed along Russia's western border.
About 1,000 of a promised 4,000 troops arrived in Poland at the start of the week, and a formal ceremony to welcome them was to be held on today.
Some people waved and held up American flags as the troops, tanks and heavy armoured vehicles cross into south-western Poland from Germany, according to the Associated Press. But their arrival was not universally applauded.
In Moscow Vladimir Putin's spokesman Dmitry Peskov said: "We perceive it as a threat. These actions threaten our interests, our security.
Especially as it concerns a third party building up its military presence near our borders. It's [the US], not even a European state.
"The Kremlin may hold back on retaliatory action in the hope that a Donald Trump presidency will herald a rapprochement with Washington. Trump, in remarks during the election campaign and since, has sown seeds of doubt over the deployments by suggesting he would rather work with than confront Putin.
But on Thursday Nato officials played down Trump's comments, saying they hoped and expected that he would not attempt to reverse the move after he became president on 20 January.
That prediction was reinforced by Trump's proposed defence secretary, General "Mad Dog" Mattis, and proposed secretary of state Rex Tillerson, who backed Nato during Senate confirmation hearings.
Mattis, in rhetoric at odds with the president-elect, said the West should recognise the reality that Putin is trying to break Nato.
Tillerson, who has business dealings in Russia, described Russia's annexation of Crimea as "as an act of force" and said that when Russia flexes its muscles, the US must mount "a proportional show of force".
Nato was caught out by the Russian annexation of Crimea from Ukraine in 2014 and has struggled to cope with Russia's use of hybrid warfare, which combines propaganda, cyber warfare and the infiltration of regular troops disguised as local rebels.
In response, the US and its Nato allies have been steadily increasing air patrols and training exercises in eastern Europe.
The biggest escalation is the current deployment of US troops, agreed at last summer's Nato summit in Warsaw.
The move was billed as an attempt to reassure eastern European states who have been calling for the permanent deployment of US troops in the belief that Russia would be less likely to encroach on territory where US troops are present.
Peter Cook, the Pentagon press spokesman, said: "The United States is demonstrating its continued commitment to collective security through a series of actions designed to reassure Nato allies and partners of America's dedication to enduring peace and stability in the region in light of the Russian intervention in Ukraine."
Poland in particular has pressed for a permanent US troop deployment since soon after the fall of communism in 1989. Nato officials insist that the US and other alliance troops deployed to eastern Europe are not "permanent", which would be in breach of an agreement with Russia.
The US plans to rotate the troops every nine months, so it can argue they are not in breach of the Russian treaty, but effectively there will be a permanent presence.
Deployment was originally scheduled for later in the month but a decision was made last month to bring it forward, possibly a move by Barack Obama before he leaves office to try to lock the president-elect into the strategy.
The troops from the Third Armor Brigade Combat Team, 4th Infantry Division, based at Fort Carson in Colorado, along with hundreds of armoured vehicles and tanks, were moved from the US to Germany last week for transit by rail and road to Poland and elsewhere in eastern Europe.
The US is sending 87 tanks and 144 armoured vehicles.
As well as being stationed in Poland, the US troops will fan out across other eastern European states, including Estonia and Bulgaria and Romania.
The UK is also contributing to the buildup of Nato forces in eastern Europe.
The UK formally took command this week of Nato's response force, made up of 3,000 UK troops plus others from Nato who will be on permanent standby ready to deploy within days.
The contributing countries include the US, Denmark, Spain, Norway and Poland.
Few at Nato seriously believe that war with Russia is likely but there have been dangerous developments, with escalation on both sides, including a build-up of Russian troops.
Russia alarmed Poland and other eastern European states by moving nuclear-capable Iskander-M missiles to its naval base at Kaliningrad in the autumn.
At the time Nato regarded the move as a response to its own deployments.
The Polish foreign minister Witold Waszczykowski, voiced concern in eastern Europe that Trump might do a deal with Putin, said this week he hoped that any such reconciliation would not be at Poland's expense.
- The Observer