After the previous three failed attempts to take Monte Cassino and break the German defences of the Gustav Line, Operation Diadem was launched on the evening of 11 May using an assortment of Allied troops. The Allied troops were amassed in the area prior to the offensive, following a campaign of dummy communications that led the Germans to believe an attack was happening further north. The resulting Allied numerical advantage was key to the success of the offensive. Allied troops made progress through the Liri valley whilst the Moroccan Goumier troops, who were adept in the mountainous terrain, managed to scale the mountains to the south and provide covering fire. The Polish troops who were tasked with attacking directly towards Monte Cassino were embroiled in bitter fighting with German paratroops for over a week until they secured the abbey on 18 May, by which point the Germans had withdrawn to the Hitler Line defences.