Tompkins entered West Point on 14 June 1911, and graduated on 12 June 1915, ranked 16th in the class of 1915, which also included
Dwight D. Eisenhower and
Omar Bradley. The class became known as
the class the stars fell on because so many of its members eventually wore the stars of a
general officer. Tompkins played
polo on the academy team. He was commissioned as a
second lieutenant in the
United States Army Corps of Engineers, and was sent to the
United States Army Engineer School at
Washington Barracks, DC, as a student, and performed additional duty as junior military aide at the
White House, during which he was promoted to
first lieutenant on 1 July 1916. From 23 September 1916 to 11 January 1917, Tompkins served with the
Mexican Expedition as an officer in the
2nd Engineers, after which he returned to the Engineer School to complete his training as a military engineer. He graduated on 14 April, just eight days after the
American entry into World War I. He then became the adjutant of the 19th Engineers (Railway), with the rank of captain from 15 May 1917. The 19th Engineers went to France via England in August, and he went to the
Western Front in command of a battalion of the 56th Engineers, a searchlight unit attached to the
French 1st Army. He was awarded the
Purple Heart, and promoted to the temporary rank of
major on 10 September 1918. ==Between the wars==