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.
More top news
A 73-mile diversion will be put in place when a three-mile stretch of the A66 between Keswick and Penrith is closed for roadworks.
Mobile banking vans will be cut from eight communities in the Scottish Borders.
Emma Jesson brings you Sunday night's forecast