In October 1943, the 1st Special Service Force, the "Devil's Brigade," joined the U.S. 5th Army in Italy. At the end of November, the commander of the U.S. 5th Army, General Mark Clark, gave the American- Canadian elite unit its first combat assignment in Europe. The 1st Special Service Force was to seize key German defensive positions on Monte la Difensa and Monte la Remetanea, which blocked the main route to Rome.
The commander of the 1st Special Services Force, Major-General Robert T. Frederick, assigned a Canadian, Colonel D.D. Williamson, the task of leading the 2nd Regiment (two battalions) in the assault. Williamson decided to scale the precipitous cliffs of Monte la Difensa, which the Germans considered impassable. Throughout the night of 2-3 December 1943, the two battalions struggled up the 3,000-foot (1,000-metre) cliffs. By dawn, they had reached the summit behind the main German positions.
As the sun rose on 3 December, Canadian Lieutenant-Colonel T.C. MacWilliam led the attack on the German pillboxes (fortified machine gun positions) and artillery emplacements. The 2nd Battalion held the summit of Monte la Difensa against several German counterattacks. On 5 December 1943, the force crossed a narrow ridge and drove the German troops from their positions on Monte la Remetanea.
The 2nd Regiment of the 1st Special Service Force suffered 400 casualties in the operation. Lieutenant-Colonel MacWilliam was among the 27 Canadians killed.