This study is a partial biography of Major General Ernest N. Harmon, focusing on his military career from his West Point graduation in 1917 to his assuming command of the 2nd Armored Division in 1942. When Harmon attained division command in July 1942 he was one of the most experienced officers in the army to command an armored division. However, he is overlooked in many histories and leadership studies. The intent of this thesis is to determine what in Harmon’s professional military development prepared him to become a successful and widely acclaimed leader of armored forces in the European Theater of Operations (ETO) during World War II. Harmon’s career reflected the generation of army officers whose service began during World War I and ended just prior to or during the early years of the Cold War. However, his World War I experience was unique in that, with only eighteen months of service, he commanded the largest U.S. cavalry formation to see combat in France. Harmon’s interwar career mirrored that of most of his peers, shifting between command, staff, instructor, and student assignments. Therefore, this study also provides a snapshot era’s officer professional development... The first twenty-five years of his career prepared Harmon for combat in World War II and the occupation of Germany that followed. His career development and personal experiences forged his competence and character. He personally played crucial roles in ending three of the greatest crises faced by American forces in the ETO: Kasserine, Anzio, and the Ardennes. The units that he commanded played decisive parts in securing North Africa, seizing Rome, and penetrating the Siegfried Line into Germany. Following the war Harmon served in a variety of key positions including military governor of Czechoslovakia and the organizer and first commander of the U.S. Constabulary Force in Germany before retiring in 1947 with thirty years of military service.