After graduation, he served on the Mexican border. After the American entry into World War I, which occurred in April 1917, Woodruff, as a young captain, became a company commander in command of Company 'H' of the 2nd Battalion, 9th Infantry Regiment, part of the 2nd Infantry Division. Woodruff was among the first of his West Point classmates to enter combat in World War I in the fall of 1917, when the division was sent to the Western Front as one of the first units of the American Expeditionary Force (AEF). This combat experience during 1917–1918 served Woodruff in good stead in his later command and staff assignments during the final stages of the war and in the post-war operations. The war came to an end on November 11, 1918.