He enlisted in the Canadian Expeditionary Force in February 1916 and was sent overseas. He started with the 182 Battalion before transferring to the 116 Battalion.

Lieutenant Crabtree was awarded the Military Cross and Mentioned in Despatches for conspicuous gallantry during an important raid upon the German lines to the north of Avion, France in September 1917.

During this raid, he led a bombing party under heavy fire, and in hand-to-hand fighting killed one man and captured two. He was 23 years old at the time.