Lassiter served with the 4th Field Artillery in Texas City, Texas from March 1913 to April 1914. He participated with his regiment in the 1914 Occupation of Veracruz, Mexico, which resulted from tensions between the United States and Mexico during the Mexican Revolution. He was assigned to the 2nd Field Artillery in November 1914, and served at Fort Stotsenburg, Philippines until September 1916. In July 1916, Lassiter was promoted to colonel.