The U.S. Army traces its history to the American Revolutionary War. The formation of the Continental Army on June 14, 1775 is considered its official "birthday." The Continental Army was disbanded in 1784. However, conflicts between western settlers and Native Americans lead to the creation of the First American Regiment. After fighting several battles with Native Americans in the ensuing decades, the unit became the 1st Infantry. In 1815, several units, including the 1st Infantry, were combined to form the 3rd Infantry. So the modern 3rd Infantry is the only unit that can trace its lineage directly back to the formation of the U.S. Army.
Re-enactors from the Kansas Army National Guard represent four periods of conflict in the history of the U.S. Army during the 231st Army Birthday ceremony in June 2006: the Revolutionary War, the Civil War, World War II and the Global War on Terrorism.
The Army was heavily involved in every U.S. military conflict in the 19th century. During the American Civil War, the U.S. Army became the Union Army. In the 20th century, U.S. Army soldiers took part in both World Wars, the Korean War, the Vietnam War and the first Persian Gulf War, as well as numerous smaller scale conflicts. Terrorist attacks and threats in the 21st century led the Army into Afghanistan and Iraq.
Because the National Guard is descended from various state militias that existed in the 1600s, it's technically older than the Army. Each National Guard unit is both a state and federal military unit. That is, the governor of each state commands the National Guard units stationed within that state, but the federal government can call on the units and bring them into action when needed. Federal authority supersedes state authority in such a situation. There are both part and full-time soldiers in the National Guard.
Since the end of the Vietnam War, the United States has held to a military doctrine called the Total Force Policy. This doctrine states that every branch of the military should be treated as a single force. Strategically, this means that the planning and deployment of the Air Force, Navy, Coast Guard and Marines should be intertwined with that of the Army. The policy is also designed to ensure the support of the American people when the nation goes to war, because the Army would be unable to effectively go to war without also activating the National Guard and Army Reserves. These two branches of the Army are considered more strongly associated with the average American citizen, especially the National Guard.
For lots more information about the U.S. Army and related topics, check out the links on the next page.