The operation marks the first time Western forces are being deployed on a continuous basis to NATO's eastern border.
Last summer, North Atlantic Treaty Organisation leaders approved the troop rotation plans in Poland and the Baltic states to reassure them that they would not be left alone if Russian Federation repeated its intervention in Ukraine.
It is hard to believe that 5,000 American troops would be a threat to Russia's security, inasmuch as Russian Federation has something like 330,000 troops along its western border.
Hailing from Fort Carson, Colorado, the so-called "Iron Brigade" is made up of highly trained soldiers and heavy military equipment.
"Welcome to Poland", Prime Minister Beata Szydlo told U.S. troops in Zagan, the Polish town on the German border where the brigade will be headquartered, adding "we hope you feel at home".
"America will always stand with Poland to defend freedom", US ambassador Paul W. Jones said Saturday.
"This is an important day for Poland, for Europe, for our common defense", Polish Prime Minister Beata Szydlo said, the AP reported.
Polish PM Beata Szydło at the ceremony on Saturday.
"These actions threaten our interests, our security", President Vladimir Putin's spokesman said on January 12.
"It's a third country that is building up its military presence on our borders in Europe", he said.
The deployment was ordered by the outgoing Obama administration and comes a week before Donald Trump's inauguration. "Americans think about the world and relations with Russian Federation much like we do".
Major General Timothy McGuire of the US Land Forces in Europe team said the thousands of soldiers would "stand united on Polish soil to deter and defend".
Along with the troop increase, 2,400 pieces of military equipment including tanks, armored fighting vehicles, artillery, trucks and containers have been deployed.
While on a nine-month deployment, the USA troops will carry out training and exercises in Poland, Estonia, Latvia, Lithuania, Romania, Bulgaria and Hungary, the embassy said.
Russia's annexation of Crimea and support for insurgents in eastern Ukraine put on edge many countries in Eastern Europe and the Baltic states that were once under Soviet control.