I. Text research Definition: A military officer, a member of the U.S. Armed Forces, is responsible for the protection of U.S. citizens. This is a broad responsibility however and each officer has a distinct role that (s) he plays in the maintenance of national defense. As a member of the military, an officer signs a contract and must serve a number of years before retirement is possible. There are five branches of the U.S. Armed Forces. The Army is the senior service and fight mostly on land. They had around 572,000 personnel in 1993. The Navy is seaborne the force of the U.S. Naval officers and enlisted spend long periods at sea and travel all about the world.