8,000 British soldiers are heading to eastern Europe for military exercises, in one of the biggest deployments since the Cold War.
It's designed to combat Russian aggression, with the UK defence secretary saying it shows "solidarity and strength".
Meanwhile in Ukraine, President Zelenskyy has given details of a recent missile strike.
It happened while the United Nations chief was visiting.
He says it shows the country can't "relax" yet.
"After our talks in Kyiv, Russian missiles flew to the city," the president added.
"This says a lot about Russia's attitude to global institutions."