Four thousand NATO troops are exercising alongside local forces across Estonia, Lithuania, Latvia, and Poland.
The former Warsaw Pact nations feel under threat from Russia which has annexed the Crimea and is widely believed to backing rebel forces fighting the government in Ukraine.
The NATO presence is described as 'enhanced forward presence', and is intended to reassure local populations as well as deter any threats from potential adversaries. The NATO Secretary General Jens Stoltenberg says the military build up is not intended to be provocative.
"We don't seek confrontation. We don't want a new Cold War."
The UK is providing the lead for NATO forces in Estonia where it has committed 800 troops along with armour and equipment.
The Canadians are leading in Latvia, the Germans in Lithuania, and the US, who have the biggest presence of any NATO country, in Poland.