As Chief of Staff of the Army, George C. Marshall ordered the largest military expansion in U. S. history, inheriting an outmoded, poorly-equipped army of 189,000 men and, partly drawing from his experience teaching and developing techniques of modern warfare as an instructor at the Army War College, coordinated the large-scale expansion and modernization of the U. S. Army. Though he had never actually led troops in combat, Marshall was a skilled organizer with a talent for inspiring other officers.
During World War II, Marshall was instrumental in preparing the U.S. Army and Army Air Forces for the invasion of the European continent. Marshall wrote the document that would become the central strategy for all Allied operations in Europe. It was assumed that Marshall would become the Supreme Commander of Operation Overlord (The “D-Day” Normandy Invasion), but Roosevelt selected Dwight Eisenhower as Supreme Commander. The President told Marshall: "I didn't feel I could sleep at ease if you were out of Washington."