American soldiers are rolling into Poland, fulfilling a dream many Poles have had since the fall of communism in 1989 to have US troops on their soil as a deterrent against Russia.
US army vehicles and soldiers in camouflage crossed into south-western Poland on Thursday morning from Germany and were heading for Zagan, where they will be based.
The US and other Western nations have carried out exercises on Nato's eastern flank, but this US deployment will be the first continuous deployment to the region by a Nato ally.
Despite the celebrations, a cloud also hangs over the historic moment: anxieties that the enhanced security could eventually be undermined by the pro-Kremlin views of President-elect Donald Trump.
Poland and the Baltic states are nervous about Russian assertiveness displayed in Ukraine and Syria.
Some people waved and held up American flags as US troops in tanks and other vehicles crossed into south-western Poland from Germany.
Poland's prime minister and defence minister will welcome them in an official ceremony on Saturday.
"This is the fulfilment of a dream," said Michal Baranowski, director of the German Marshall Fund think tank in Warsaw.
"And this is not just a symbolic presence but one with a real capability."
It is part of a larger commitment by President Barack Obama to protect a region that grew deeply nervous when Russia annexed Crimea from Ukraine in 2014 and then began backing separatist rebels in Ukraine's east.
Poland and the Baltic states also feel threatened by Russia's recent deployment of nuclear-capable Iskander missiles in Kaliningrad, the Russian territory wedged between Poland and Lithuania.
But Russia says it is the one who is threatened.
"These actions threaten our interests, our security," President Vladimir Putin's spokesman Dmitry Peskov said.
"Especially as it concerns a third party building up its military presence near our borders. It's not even a European state."
Worries about the permanence of the new US security commitments are rooted in a tragic national history in which Poland has often lost out in deals made by the great powers.
Poles still feel betrayed by Mr Obama's "reset" with Russia early on in his administration, which involved abandoning plans for a major US missile defence system in Poland and replacing it with plans for a less ambitious system, still not in place.
"All recent US presidents have thought there can be a grand bargain with Russia," said Marcin Zaborowski, a senior associate at Visegrad Insight, an analytic journal on Central Europe.
"Trump has a proclivity to make deals, and Central and Eastern Europe have reason to worry about that."
Polish foreign minister Witold Waszczykowski expressed hope this week that any new effort at reconciliation with Russia "does not happen at our expense".
The armoured brigade combat team arriving in Poland hails from Fort Carson, Colorado.
The troops arrived last week in Germany and are gathering in Poland before units will fan out across seven countries from Estonia to Bulgaria.
A headquarters unit will be stationed in Germany.
After nine months they will be replaced by another unit.
In a separate but related mission, Nato will also deploy four battalions to its eastern flank later this year, one each to Poland and the three Baltic states.
The US will also lead one of those battalions.