Polish leaders are holding a welcome ceremony for the biggest deployment of US troops to Europe in decades.
The ceremony on Saturday in the western Polish town of Zagan comes 23 years after the last Soviet troops left the country.
Russia called the deployment a threat to its own security.
The move also marks a new historic moment - the first time any Western forces are being deployed on a continuous basis to NATO's eastern flank.
Polish Prime Minister Beata Szydlo stood before US and Polish soldiers and called it "an important day for Poland, for Europe, for our common defence".
Defence Minister Antoni Macierewicz told the troops that "we waited for you for a very long time, for decades".
The government organized several other events across the country to welcome the Americans from Fort Carson, Colorado.