Crabtree attended Stonyhurst College. On 5 September 1914, a matter of days after the outbreak of the First World War, Crabtree enlisted as a private in the Loyal North Lancashire Regiment. Just over a year later, he was commissioned as a second lieutenant. Crabtree was wounded near Morval during the Battle of the Somme in September 1916 and saw further action at Arras and Passchendaele after his recovery. He won a Military Cross for "conspicuous gallantry and devotion to duty" on 26 September 1917. A month later, he was wounded for a second time and did not serve on the Western Front for the remainder of the war. After being promoted to captain, Crabtree was posted briefly to Central Asia before leaving the army.