The battalion is one of four NATO is deploying for the first time to Poland and the Baltic states as tripwires against Russian adventurism on its eastern flank, a region formerly under Moscow's control and spooked by its actions in Ukraine.
"Generations of Poles have waited for this moment since the end of World War II, generations that dreamt of being part of the just, united, democratic and truly free West," President Andrzej Duda said at ceremonies in the northeastern Polish town of Orzysz.
Poland joined NATO in 1999, a decade after it peacefully shed communism as the Iron Curtain fell in 1989.
Speaking along side Duda, NATO Supreme Allied Commander US General Curtis Scaparrotti called the deployment "a clear demonstration of NATO's unity and resolve and sends a clear message to any potential aggressor."
"You now form NATO's eastern flank," he told troops.
Three similar NATO units, led by the Germany, Canada and Britain, are being deployed this year in Lithuania, Latvia and Estonia. Each unit includes around 800 troops.
The countries requested them after Russia's 2014 annexation of Crimea from Ukraine.
The Kremlin has denied any territorial ambitions and claims that NATO is trying to encircle Russia.
But Moscow's deployment last year of nuclear-capable Iskander missiles into its Kaliningrad exclave, which borders Lithuania and Poland, and frequent Russian military drills in the region have rattled nearby NATO members.
The Orzysz NATO base lies around 100 kilometres (60 miles) from the Kaliningrad exclave and a stone's throw from the strategically sensitive "Suwalki Gap", a land corridor critical to the security of the Baltic states.
The 65-kilometre stretch of border with Lithuania is sandwiched between Kaliningrad and Belarus.
Military strategists warn it is the Achilles' heel of NATO's eastern flank: its capture would amputate the alliance's three Baltic members and so shatter its credibility.
The freshly deployed NATO troops are expected to hold manoeuvres around the Suwalki Gap in June.
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