After the attack on Lee’s Cold Harbor line was bloodily defeated on June 3, Grant realized that trying to bull his way through Lee’s fortified line was useless. As he had throughout the Overland Campaign Grant decided to move again south and east around Lee’s left. He chose to not only cross the Chickahominy River but also the James River, a move he hoped would take Lee completely by surprise and allow him to seize Petersburg, the rail hub supplying Richmond.
To divert Lee’s attention he sent Sheridan and most of his cavalry on a raid to the West. Grant then began the construction of an entrenchment line behind his Cold Harbor position. On the night of the 12th Hancock’s and Wright’s corps withdrew to the new entrenchments. Warren’s corps crossed the Chickahominy River and headed south. Burnsides corps followed with Hancock and Wright’s corps taking up the rear. Smith’s corps marched to White House on the Pamunkey River and were shipped by the navy to Bermuda Hundred.
At 4:00 PM on June 15th Union engineers began work on a 2200 feet pontoon bridge on the James between Windmill Point to Fort Powhatan and completed it seven hours later. Grant then crossed his army over the James during the next two days with Lee still unsure as to his intentions, in one of the most daring, and successful, maneuvers of the War. Grant in his Memoirs describes why he decided to take his biggest gamble of the War:
LEE’S position was now so near Richmond, and the intervening swamps of the Chickahominy so great an obstacle to the movement of troops in the face of an enemy, that I determined to make my next left flank move carry the Army of the Potomac south of the James River. Preparations for this were promptly commenced. The move was a hazardous one to make: the Chickahominy River, with its marshy and heavily timbered approaches, had to be crossed; all the bridges over it east of Lee were destroyed; the enemy had a shorter line and better roads to travel on to confront me in crossing; more than fifty miles intervened between me and Butler, by the roads I should have to travel, with both the James and the Chickahominy unbridged to cross; and last, the Army of the Potomac had to be got out of a position but a few hundred yards from the enemy at the widest place. Lee, if he did not choose to follow me, might, with his shorter distance to travel and his bridges over the Chickahominy and the James, move rapidly on Butler and crush him before the army with me could come to his relief. Then too he might spare troops enough to send against Hunter who was approaching Lynchburg, living upon the country he passed through, and without ammunition further than what he carried with him.
But the move had to be made, and I relied upon Lee’s not seeing my danger as I saw it. Besides we had armies on both sides of the James River and not far from the Confederate capital. I knew that its safety would be a matter of the first consideration with the executive, legislative and judicial branches of the so-called Confederate government, if it was not with the military commanders. But I took all the precaution I knew of to guard against all dangers.