A large military training exercise involving more than 20 NATO and partner countries began in Poland amid growing efforts to reassure eastern European nations rattled by Russia's actions in nearby Ukraine.
For more than 10 days of the month of June, 30,000 troops, backed by large numbers of vehicles, aircraft and ships, will be deployed in one of the biggest exercises on NATO's eastern flank since the end of the Cold War.
The move is likely to put further strain on the tense relations between the Kremlin and the West.
The Anakonda-16 exercise, which includes manoeuvres such as a night-time helicopter assault and the dropping of US paratroopers to build a temporary bridge over the Vistula river, is being held one month before a NATO summit in Warsaw that will approve that more troops are stationed in eastern Europe.
The goal of Anakonda-16 is to "train, exercise and integrate the Polish national command and force structures into an allied, joint multinational environment", the US Army Europe said.
The United States will provide about 14,000 troops for the exercise, the largest foreign contingent. Non-NATO countries such as Sweden and Finland are also taking part in the exercise.
Poland joined NATO in 1999, a decade after the demise of Moscow-backed communism in eastern Europe. Warsaw has been very critical of Moscow's actions in Ukraine and has repeatedly urged NATO to boost its presence on Polish territory.