Complex sentence: When Montcalm's errors led to the British military triumph at Quebec City under Maj. Gen. James Wolfe, Vaudreuil-Cavagnal tried to rally resistance to the British, but was forced to surrender Montreal on Sept.8,1760, to Maj. Gen. Jeffrey Amherst.
Split into simpler sentences: When Montcalm's errors led to the British military triumph at Quebec City under the leadership of Maj. Gen. James Wolfe, Vaudreuil-Cavagnal tried to rally resistance to the British, but to no avail. He was forced to surrender Montreal on Sept.8,1760 to Maj. Gen. Jeffrey Amherst.