During the U.S. Civil War (1861–1865) the Marine Corps played only a minor role after their participation in the Union defeat at the first battle of First Bull Run/Manassas. Their most important task was blockade duty and other ship-board battles, but they were mobilized for a handful of operations as the war progressed.

Before World War I, the U.S. Marine Corps wasn't the true expeditionary warfighting branch that it is today. It was frequently neglected, underfunded, and at one point entirely disbanded. The Civil War Marine Corps usually had just a little over 3,000 men but manned the guns at vital points in the Union war machine, helping usher in the victory.

Before we upset any Marines, it’s important to note that while there were just 4,000 Marines in the Corps during the Civil War, it played an extremely important role in the Union’s overall strategy, was pivotal in a number of pitched battles, and 17 Marines earned the Medal of Honor during the war.