John Butler returned to service, as a Loyalist, when the American Revolution turned to war in 1775. In May 1775, he left for Canada in the company of Daniel Claus, Walter Butler, Hon Yost Schuyler and Joseph Brant, a Mohawk leader. On July 7, they reached Fort Oswego and in August, Montreal. Butler participated in the defense of Montreal against an attack led by Ethan Allen. In November, Carleton sent him to Fort Niagara with instructions to keep the Indians neutral.