Bradley was commissioned as a second lieutenant into the Infantry Branch of the United States Army and was first assigned to the 14th Infantry Regiment. He served on the Mexico–United States border in 1915, defending it from incursions due to the Mexican civil war. When the United States entered World War I in April 1917 (see the American entry into World War I), he was promoted to captain and sent to guard the Butte, Montana copper mines, considered of strategic importance. Bradley joined the 19th Infantry Division in August 1918, which was scheduled for European deployment, but the influenza pandemic and the armistice with Germany that fall intervened.