The UK government said Friday it was deploying about 8,000 troops for exercises across eastern Europe in a show of strength after Russia's invasion of Ukraine.
The months-long drills in countries stretching from Finland to North Macedonia also involve allies including France and the United States, defence officials said in a statement.
"The security of Europe has never been more important," Defence Secretary Ben Wallace said, calling it "one of the largest shared deployments since the Cold War".
The officer in command, Lieutenant General Ralph Wooddisse, said Britain was making "a significant contribution to the defence of Europe and the deterrence of Russian aggression".
"The British Army's series of exercises is fundamental to both."
Britain has been supplying missiles to Ukraine to fight back against Russia, and says it is ready to help with tanks and planes via partners such as Poland.
Foreign Secretary Liz Truss said Wednesday that "the whole of Ukraine" must be liberated, including Crimea, after the peninsula was seized by Russia in 2014.