Complex sentence: Rawlinson was kept reined in and advised by Haig to ensure his men were well rested for the eventual attack on the Line; however, when news arrived of the British Third Army's victory at the Battle of Havrincourt, Haig's mind was changed.
Split into simpler sentences: Rawlinson was kept reined in and advised by Haig to ensure his men were well rested for the eventual attack on the Line. When news arrived of the Third Army victory at the Battle of Havrincourt, Haig's mind was changed.