 Mackay was given command of the 1st Machine Gun Battalion, the 1st Division's new battalion, which he led in the Battle of Hazebrouck.He was given the brevet rank of major in the 26th Infantry Battalion in the AMF back home on 3 June 1918.On 6 June 1918, Mackay was heading on leave to London to visit his wife, who had managed to reach England after a long battle with wartime travel restrictions, when he was stopped and turned back at Boulogne by British military police.