At the outbreak of the Civil War, William Wells and three of his brothers joined the Union army. Wells enlisted as a private soldier on September 9, 1861, and assisted in raising Company C of the 1st Vermont Cavalry. He was sworn into Federal service October 3, 1861, and was soon promoted first lieutenant and then captain in November of that year. He was in the thickest of the fight at Orange Court House, Virginia, August 2, 1862, and was promoted to major on October 30, 1862.
Big Round Top cavalry charge
Part of Battle of Gettysburg, third day cavalry battles
Gettysburg South Cavalry Field.png
The Wells cavalry charge rode eastward across Plum Run and along a stone wall to where "the troopers reached the spur" of Big Round Top and turned north to pass to the rear of Law's Alabama regiments. The Confederates turned about and fired on the cavalry: "It was a swift, resistless charge, over rocks, through timber, under close enfilading fire." (Captain Henry C. Parsons)[2] After "a struggle, the hill was carried by the 1st Vermont",[3] and in 1891, Wells received the Medal of Honor "for leading the second battalion of [the 1st Vermont] regiment on a daring charge" at Gettysburg.[4]
Date    July 3, 1863 (5 pm)[5]: 10 
Location    
Gettysburg Battlefield, Pennsylvania
Result    hill "carried by the 1st Vermont",[3]
1891 Wells' Medal of Honor
 
Belligerents
United States USA (Union)    Confederate States of America CSA (Confederacy)
Commanders and leaders
Major William Wells
Strength
2nd Battalion, 1st VT[3]    5 regiments of Law's Brigade[3]
Following Wells' 1891 Medal of Honor for the charge,[4] his statue was erected on the 1913 1st Vermont monument.
Medal of Honor
Wells commanded the Second Battalion, 1st Vermont Cavalry, in the repulse of Stuart's Cavalry at the Battle of Hanover during the Gettysburg Campaign. In the famous and desperate cavalry charge on Big Round Top on the third day at Gettysburg (July 3, 1863), he commanded the leading battalion, rode by the side of General Farnsworth, the brigade commander, and, almost by a miracle, came out unharmed, while his commander fell in the midst of the enemy's infantry. A few days later, in the savage cavalry melee at the Battle of Boonsboro in Maryland, Wells was wounded by a sabre cut. At Culpeper Court House, Virginia, September 13, 1863, he charged the enemy's artillery with his regiment and captured a gun, and was again wounded, by a shell. Congress later awarded Wells a Medal of Honor "for distinguished gallantry at the battle of Gettysburg, July 3, 1863."
After the return of the regiment from the Kilpatrick raid, in March 1864, Major Wells was detached and placed in command of the 7th Michigan Cavalry (which had lost its commander) for a month. He commanded a battalion of Sheridan's cavalry at the Battle of Yellow Tavern, in which J.E.B. Stuart was killed. Wells was promoted to colonel on June 4, 1864. From September 1864 to April 1865, he was several times in command of the Third Cavalry Division. In the Battle of Tom's Brook, Virginia (October 9, 1864), a cavalry action, Wells commanded a brigade of Custer's division, and at Cedar Creek, his brigade took a foremost part in turning the rout of the morning into a decisive victory at nightfall, capturing forty-five of the forty-eight pieces of artillery taken from Jubal Early's fleeing army.

Plaque of General Wells's military record (Battery Park, Burlington, VT)
Wells was appointed brevet brigadier general of volunteers on February 22, 1865. On the personal recommendations of Generals Sheridan and Custer, Wells was commissioned brigadier general May 16, 1865, and he was appointed brevet major general of volunteers, March 30, 1865, "for gallant and meritorious service," having received more promotions than any other Vermont officer during the war (from Private to General in less than three and a half years). Wells served under Generals Kilpatrick, Sheridan, and Custer, and was with Kilpatrick in his famous raid on Richmond, and with James H. Wilson in his daring foray to the south of that city. At Appomattox Court House, on the morning of the surrender of the Army of Northern Virginia, Wells' brigade had started on its last charge and was stopped by General Custer in person. He distinguished himself repeatedly in action. Sheridan remarked: "He is my ideal of a cavalry officer."
Following Appomattox, the departure of Sheridan and Custer for Texas left Wells as the ranking officer and last commander of the Cavalry Corps of the Army of the Potomac. At the Grand Review in Washington, D.C. on May 22, 1865, he commanded the Second Brigade of Custer's Division of the Cavalry Corps, which led the advance.
Wells participated in seventy cavalry engagements, in eighteen of which he led a brigade or division. He was honorably mustered out of army January 15, 1866.