The 5th Corps of the Union Army of the Potomac with its famous Maltese cross was organized May 18, 1862, while engaged in the Peninsular campaign. The first battle of the Corps occurred at Hanover Court House, Virginia, May 27, 1862, an engagement in which one of its Divisions stood the brunt of the fighting, and won a creditable victory. The following battle of Gaines' Mill was fought, almost entirely, by the 5th Corps; the troops holding their position stoutly, although the attacking forces comprised almost the entire Confederate Army. The losses of the Corps in the Seven Days Battle were half the entire losses of the Union Army; a significant baptism by fire.

Its next battle was Second Bull Run, where the corps did some of the best fighting on that field. The largest regimental loss, in killed and wounded, in the entire Union Army, occurred in the Duryeé Zouaves, one of the 5th Corps regiments.

The Corps continued to distinguish itself at Antietam, Fredericksburg (where they attempted to carry Marye's Heights after all other efforts had failed and were also slaughtered by the Confederates defending the heights), and Chancellorsville by which time they were under the command of General Meade. At Gettysburg the Corps distinguished itself in battle by its fighting in the wheat-field, and also by the gallant action of Vincent's Brigade in seizing Little Round Top, just in time to save the Army from what might have been a devastating disaster. The famed defense by the 20th Maine Volunteer Infantry Regiment of Vincent's Brigade commanded by Colonel Joshua Lawrence Chamberlain is the reason we proudly fly the pendant with the Maltese cross. Chamberlain's now famous orders he was given to defend to "the last" and the heroic bayonet charge after his command had run out of ammunition and more than a third of his men had fallen, lives in memory as one of the most significant and courageous decisions ever made in the heat of the battle.

The Corps and Chamberlain fought heroically at the battles of the Wilderness, Spotsylvania, Cold Harbor and the siege of Petersburg until the last battle at Five Forks. Chamberlain was assigned to brigade command in June 1864, only to fall wounded 12 days later leading his men in the assault on Petersburg, he was promoted to brigadier general on the spot by General Grant, and then carried to the rear to die. After a surgeon performed a two-hour effort to extract a bullet from his abdomen, without anesthetic, he miraculously survived and at the surrender of Lee's Army of Northern Virginia at Appomattox Court House, Grant chose Brevetted Major General Chamberlain to receive the formal surrender of weapons and colors (12 April 1865). Chamberlain had his men salute the defeated Confederates as they marched by, evidence of his admiration of their valor. Chamberlain was awarded the Congressional Medal of Honor for the day at Gettysburg, was wounded six times, had six horses shot from under him, and eventually died of complications from his Petersburg wounds in 1914 at the age of 85. The proud 5th Corps was discontinued, June 28, 1865, but her colors still fly.