American citizenship and practice of war have evolved significantly since World War II. In the years following the Vietnam War, starting with the end of the draft, the Armed Forces of the United States formed a "military cluster"; that is, a professional, long-serving fighting force with its own unique set and system of values, ethics, and beliefs. During the two long wars in Afghanistan and Iraq, the government used reenlistment bonuses and other financial incentives to keep soldiers and marines on active duty, and to entice new recruits to join. Those with least are the most responsive to financial incentives. However, patriotism and love of service still matter. It motivates and inspires men and women in uniform. The current American practice of war is unsustainable. This introduction presents an overview of the key concepts covered in the subsequent chapters of this book. The book analyzes the American system and practice of war from World War II to Operation Enduring Freedom.