Polish President Andrzej Duda has hailed the activation of the US-led battalion in the region bordering the heavily militarized Russian exclave of Kaliningrad. "Generations of Poles have waited for this moment," he said.
The NATO battalion is one of four to be deployed in Poland and the Baltic states, serving as "tripwires" on the eastern flank of the alliance. The Eastern European nations asked for support after Russia annexed Crimea in 2014.
Welcoming over 1,000 troops on Thursday, Polish President Duda said it was a "historic moment" for his country.
"Generations of Polish people have waited for this moment since the end of the Second World War, dreaming about Poland's return to membership in the just, solidary, democratic and truly free West," Duda said at the town of Orzysz.
The new Orzysz base is located some 60 kilometers (37 miles) from the border of Russian Kaliningrad exclave. The battalion is led by the US military, but also includes troops from Britain and Romania, with Croatian troops expected to arrive later.
In a separate deployment earlier this year, the US sent 3,500 troops to a Polish base near the German border.
Poland looking to west
In a 1997 agreement with Russia, NATO pledged not to create permanent bases in former Eastern bloc states "in the current and foreseeable security environment." After the Crimean crisis, however, NATO decided to boost its presence with troops that would be rotated rather than stationed permanently. The Russian side sees this as a breach of the deal and a threat to its security.
The exclave of Kaliningrad borders Poland and Lithuania. Russia has recently moved nuclear-capable Iskander missiles to the territory, which also serves as headquarters for the country's Baltic Fleet.
Poland became part of the Soviet-dominated Warsaw pact after World War II, but joined NATO in 1999, ten years after the Iron Curtain fell. The anti-Russian sentiment is still strong in the country ruled by the right-wing PiS government.