The invasion of Sicily, was the first part of the ambitious plan to invade Italy. It was a massive logistical effort, second only to the D-Day landings. The plan included two armies, the US Seventh, and the British Eighth Army. The Germans, under the command of Field Marshal Albert Kesselring, used the mountainous terrain of both Sicily and Italy to make life as difficult as possible for the two armies. Hitler had ordered that Sicily be kept at all costs. However, Keselring soon realised that this was not viable if the Germans wanted to protect their valuable weapons and equipment. In August, 1943, they withdrew 40,000 troops and 60,000 Italin troops back across the Straits of Messina to the Italian mainland.