In May 1942, Truscott was assigned to the Allied Combined Staff under Lord Louis Mountbatten and in August, he was the primary U.S. observer on the Dieppe Raid. The raid was primarily a Canadian operation, consisting of elements of the 2nd Canadian Infantry Division, with two British Commandos attached along with a 50-man detachment from the 1st Ranger Battalion. The Rangers were assigned to No. 3 Commando, No. 4 Commando, and 6 Rangers were spread out among the Canadian regiments. This was considered the first action by American troops against German forces in World War II.