I Corps (Union army)

There were four formations in the Union Army designated as I Corps (or First Army Corps) during the American Civil War.

Three were short-lived:

• In the Army of Virginia, a command later known as XI Corps (Army of the Potomac) and (Army of the Cumberland):
  • John C. Frémont (June 26 – June 29, 1862)
  • Franz Sigel (June 29 – September 12, 1862)
• In the Army of the Ohio and Army of the Cumberland:
  • Alexander M. McCook (September 29 – November 5, 1862)
• In the Army of the Mississippi:
  • George W. Morgan (January 4 – 12, 1863)

The other, the I Corps, Army of the Potomac, also temporarily known as III Corps, Army of Virginia, is the subject of this article.

The term "First Corps" is also sometimes used to describe the First Veteran Corps from 1864 to 1866.