Who was the only Marine officer to command a field Army in history?

Major General Roy S. Geiger is recognized as the only Marine officer to command a field Army in history. This significant achievement occurred during World War II when he served as the commander of the 3rd Marine Amphibious Corps, which was a key component of the ground forces in the Pacific Theater. His capability to command a larger Army formation illustrates the unique versatility and leadership potential of Marine officers, particularly in joint operations alongside Army units.

In addition to his command of the field Army, Geiger's military career included notable contributions to amphibious warfare and coastal operations, further solidifying his legacy within the Marine Corps and the larger U.S. military. His successful leadership during critical campaigns showcases the high level of training and operational readiness of Marine Corps officers in joint warfare contexts.