Grant continued to move south and Lee reinforced Richmond with much of his army to block him once more but Grant’s objective was Petersburg which was then defended by less than three thousand troops.  On June 12, 1864, the Army of the Potomac began to cross a 2,100 foot long pontoon bridge over the James River. General W.F. “Baldy” Smith commanded the 16,000 troops that were the first Union troops to arrive outside Petersburg on June 15^th but he did not launch an assault despite the urging of General Winfield Scott Hancock. This delay allowed Confederate reinforcements to man the Petersburg defenses and prolonged the Civil War until April 1865.

Petersburg became a siege and the concept of trench warfare was born. The Union army lived in a network of trenches, suffering from the harsh elements, open to artillery bombardment and small arms fire. Union soldiers that were  Pennsylvania coal miners dug a tunnel under a fort in the middle of the Confederate line  and  four tons of gunpowder were  placed under the fort. In the early morning of July 30, 1864, the explosion was the signal for Union forces to attack through the resulting breach in the Confederate lines. The assault was initially successful, but many Union troops were trapped in the 170 foot deep crater without ladders to climb out and the Confederate defenders quickly regrouped and poured small arms and artillery fire into the crater. The resulting slaughter caused almost 4,000 Union casualties. Many black regiments were engaged in the attack through the crater, including the 31^st United States Colored Troops containing some Mansfield soldiers. The 21^st Regiment Connecticut Volunteer Infantry were ordered to commence firing on the enemy from their trenches on the Petersburg when the mine exploded. 

After this unsuccessful assault, Grant continued to expand his lines, leaving the undermanned Confederates to thinly cover their defenses in response. The outer defenses of Petersburg were not taken until April 2, 1865.  Lee was then forced to order the evacuation of Petersburg and Richmond.