In the last week of March 1941, a German-led force launched an offensive in Cyrenaica which rapidly defeated the Allied forces in the area, forcing a general withdrawal towards Egypt (April 1941). The 9th Division formed the rear guard of this withdrawal, and on 6 April, was ordered to defend the important port town of Tobruk for at least two months. During the ensuing siege of Tobruk the 9th Division, reinforced by the 18th Brigade of the 7th Division and British artillery and armoured regiments, used fortifications, aggressive patrolling and artillery to contain and defeat repeated German armoured and infantry attacks. The Mediterranean Fleet sustained Tobruk's defenders, and the elderly Australian destroyers made repeated supply "runs" into the port. and were sunk during these operations. Upon the request of the Australian Government, the bulk of the 9th Division was withdrawn from Tobruk in September and October 1941, and was replaced by the British 70th Division. The 2/13th Battalion was forced to remain at Tobruk until the siege was lifted in December when the convoy evacuating it was attacked, however. The defence of Tobruk cost the Australian units involved 3,009 casualties, including 832 killed and 941 taken prisoner.