As Allied forces in Malaya retreated towards Singapore, a 2,000-strong detachment of 8th Division reinforcements arrived in Singapore, including the 2/4th Machine Gun Battalion. These reinforcements were largely provided to the 2/19th and 2/29th Battalions which had suffered heavy casualties in Malaya, although most had not completed basic training and they were ill-prepared for the fighting to come. By 31 January, the last British Commonwealth forces had left Malaya, and engineers blew a hole wide in the causeway. The Allied commander, Lieutenant-General Arthur Percival, gave Major General Gordon Bennett's 8th Division the task of defending the prime invasion points on the north side of the island, in a terrain dominated by mangrove swamps and forest. The 22nd Brigade was assigned a daunting wide sector in the west of the island amidst a tangle of islets and mangrove swamps, and the 27th Brigade a zone in the north-west, near the causeway. From vantage points across the straits, including the Sultan of Johore's palace, as well as aerial reconnaissance and infiltrators, the Japanese commander, General Tomoyuki Yamashita and his staff gained an excellent knowledge of the Allied positions. From 3 February, the Australian positions were shelled by Japanese artillery. Shelling and air attacks intensified over the next five days, destroying communications between Allied units and their commanders.