Exercise Swift Response 2019 involves 600 British personnel (Picture: MOD).
Six hundred personnel from 16 Air Assault Brigade and the 11th Infantry Brigade are in Croatia for Exercise Swift Response.
They will train alongside American and French allies, with 25 countries in total supporting, or deploying on, the exercise.
The British Army and the Royal Air Force will also contribute four Chinook, four Wildcat and six Apache helicopters to the exercise.
Minister for the Armed Forces Mark Lancaster said: "Last week paratroopers re-enacting the D-Day drop into northern France provided a visual reminder of the extraordinary feats made possible when we work hand in glove with allies toward a common goal."
Exercise Swift Response will include more than 7,000 personnel – 1,000 of them parachuting – 300 vehicles and 40 aircraft.
It aims to support stability in the region, and Euro-Atlantic integration, by improving coordination amongst allies and partners.