In the spring of 1864, the ninth corps re-assembled at Annapolis, under Gen. Burnside, and Col. Griffin was assigned to the command of the second brigade, second division. On the 5th of May the corps joined the army of the Potomac, on the Rapidan, and at two o'clock on the morning of the 6th, Col. Griffin was sent with his brigade to attack the enemy, and later in the day made a brilliant charge in repelling an attack made on the second corps. At Spottsylvania Court-House, May 12, Gen. Hancock made the assault at four o'clock in the morning. Griffin occupied the right of the ninth corps, on the left of Hancock, though some distance from him, with orders to support that officer. Promptly at four o'clock Griffin advanced with his brigade in line of battle, and made directly for the point of attack indicated by the sound of Hancock's guns. As he approached, he galloped forward to see just where to make the connection. Passing out of a wood into an open field, he found Hancock's troops wild with excitement over their success, but with organizations completely broken up by the charge they had made. Looking across a valley to a slope beyond, he saw a large force of rebels advancing rapidly to make a counter attack. Hastening back to his command, he brought it forward into position just in time to take that advancing column in front and flank with a destructive fire. Other brigades came up and formed on his left, and for five hours a terrific fire was kept up, and the furious onslaught of three Confederate divisions was repulsed. The loss on each side was fearful, but Hancock's corps, and possibly the army, was saved from being swept away, and a victory was won. By this gallant act Col. Griffin "won his star," being made a brigadier-general of volunteers by President Lincoln, on the recommendation of Generals Burnside and Grant, and confirmed by the senate without debate, reference, or a dissenting vote. On the 18th he made a reconnoissance with his brigade, and handled it with coolness and skill in the fights of North Anna, Tolopotomy Creek, Bethesda Church, and Cold Harbor. On the arrival of the army in front of Petersburg, June 15, he was placed in command of two brigades, and made a skillful attack on the enemy's advanced lines at daylight next morning, capturing one thousand prisoners, fifteen hundred stands of arms, four pieces of artillery, with caissons, horses, and ammunition, and opening the way into Petersburg had supports been ready in time. At the battle of the "Mine" he commanded his brigade, and did every thing that could be done in his place to insure success; also at the Weldon Railroad, Poplar Grove Church, and Hatcher's Run.