Thousands of US troops are massing in Poland as part of a planned NATO operation in what has been touted by Washington as “defense against Russian aggression," a provocative move that recalls the hay days of the Cold War era.

Around 4,500 US troops began arriving on Monday in the southeastern city of Wroclaw, home to a key NATO and Polish air base, gearing up for upcoming drills in the military alliance’s member states near Russia.

The troops will be followed by more than 2,800 pieces of American military equipment, including tanks and armored vehicles, and they all will be in turn redeployed in five other locations in future.

The major deployment of the troops has been described as the largest movement of an armed US military brigade to Europe since the conclusion of the Cold War.

On Friday, US warships started unloading the military hardware, which includes 87 tanks and 144 Bradley fighting vehicles, in the northern German port of Bremerhaven, from where they would be transported by rail and road to Poland. The US troops first arrived in the German city before starting to move to Wroclaw.

Troops will be on rotation in Poland and beyond for between four and six months and will cooperate with local units. The US also aims to conduct a joint military exercise with Poland, which will kick off at the end of January.

Moscow has already lambasted the move as “truly aggressive.”

US military trucks and Humwees, which arrived by ship, are unloaded in the harbor of Bremerhaven, Germany, January 8, 2017. (Photo by Reuters)

The US and its allies have been at odds with Moscow since the strategic Black Sea peninsula of Crimea, controlled by Ukraine, joined Russia following a referendum in March 2014.

Western countries have been fearful of a repeat of that scenario in other countries, and have sought to step up their military presence in Eastern Europe.

The European Union, the US and some other Western countries have also imposed several rounds of sanctions against Russia.

NATO, for its part, has suspended all practical cooperation with Moscow and started to deploy troops and weaponry to Baltic States—Latvia, Lithuania and Estonia—near Russia since then.

Moscow is seriously wary of the US-led alliance’s military build-up near its borders. In response to the aggressive measures, Russia has beefed up its southwestern military capacity, deploying nuclear-capable Iskander-M missiles to its Baltic enclave of Kaliningrad bordering Poland and Lithuania.