 Complete the following phrase:  Mackay graduated from the University of Sydney in 1904 and taught physics there from 1910 until 1914, when he joined the Australian Imperial Force shortly after the outbreak of the First World War. He served with the 4th Infantry Battalion at Gallipoli, where he distinguished himself in hand-to-hand fighting at the Battle of Lone Pine. In April 1916, he assumed command of the 4th Infantry Battalion on the Western Front and led it at the 