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. He had been appointed to command the 1st Infantry Brigade and had to return at once. He was immediately promoted to colonel and temporary brigadier general. Mackay commanded the 1st Infantry Brigade in the later stages of the fighting around Hazebrouck. In the operations east of there in June and July, his brigade was exceptionally active in the form of minor operations and patrolling that became known as the peaceful penetration.