More than 3,000 troops are taking part in "Operation Joint Warrior" in and around Galloway Forest in Dumfriesshire.
A group of British and French paratroopers are also carrying out a parachute drop near to the Braid Fell area.
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Pictures from the commemoration marking the centenary of the Quintinshill rail disaster, which killed more than 200 people, is underway.
The 100th anniversary of Britain's worst rail disaster will be commemorated at a special service today.