From Grant, by Ron Chernow (Penguin, 2018), Kindle pp. 411-412:
By June 12, the weather had turned cool and windy. That night, after dark, Grant began to march his army toward the James. Staff officers noticed the tense way Grant relit cigars constantly and reacted with monosyllables. “Yes, yes,” or “Go on—go on.” On this splendid night full of moonlight, the tramp of feet lifted swirling dust that soon obscured the stars. By the next morning, in a logistical masterpiece, the Army of the Potomac had vacated the Cold Harbor trenches. Lee was completely fooled by the exodus and thunderstruck to discover that Grant’s entire army of 115,000 men had vanished in the night. While he had a hunch that Grant would swerve toward the James River, he could not be certain. To confound Lee further, Grant ordered some units to conduct diversionary feints toward Richmond.
Meanwhile, Grant’s main army crossed the Chickahominy River and reached the formidable James River barrier. Grant needed to take his massive army across a waterway two thousand feet wide and eighty-four feet deep. To Julia, he described the operation as “one of the most perilous movements ever executed by a large army” since it involved “crossing two rivers over which the enemy has bridges and railroads whilst we have bridges to improvise.” Ever the optimist, he shook off the settled gloom of Cold Harbor. “I am in excellent health and feel no doubt about holding the enemy in much greater alarm than I ever felt in my life.”
On the morning of June 14, Grant’s engineers began to span the majestic James with a pontoon bridge measuring 2,100 feet in length and 13 feet in width, making it the longest such bridge in military annals. It was anybody’s guess whether such a lengthy bridge, buoyed by 101 floats, could withstand tidal currents or gusts sweeping inland from Chesapeake Bay. Miraculously, the entire bridge was completed shortly after midnight. The next day, his hands joined behind his back, Grant gazed silently from a bluff on the river’s north side as cavalry and artillery trains moved rapidly across the river. “He wore no sword or other outward trapping except his buttons and plain shoulder straps,” one soldier had observed a day earlier. “His pants were tucked inside of a pair of long dusty boots and his whole attire looked dirty & travel stained.”
Grant officiated at one of the war’s most stirring spectacles. On this cloudless day, brilliant sunshine sparkled off the water, gun barrels, and cannon trundling across the bridge. To the crisp beat of marching bands, troops stepped briskly onto ferry boats that plied the river at a dizzying pace. Nearby gunboats kept a watchful eye on any threatening enemy movements. Before the operation was over, an enormous herd of cattle swam across the river. From the capital, Lincoln applauded Grant, telegraphing at 7 a.m. on June 15: “I begin to see it. You will succeed—God bless you all!” By around midnight the next day, the last remnants of Grant’s army had crossed the river. Incredibly, Lee still had no idea Grant’s army had slipped across the James in an operation so stupendous even one Confederate general dubbed it “the most brilliant stroke in all the campaigns of the war.”
On the day the pontoon bridge was laid down, Grant and Rawlins traveled by steamer up the James to Bermuda Hundred to consult with Ben Butler. As a general, Butler hadn’t covered himself with glory, but as a noted Democratic politician, he was too useful for Lincoln to scrap. Grant found Butler covering the Appomattox River with another amphibious bridge to carry his men on a raid into Petersburg, only six miles away. Grant hoped to take Petersburg before Lee was alerted to his whereabouts. On the evening of June 15, Baldy Smith and Winfield Scott Hancock achieved startling success when they overran the outer defensive rim of northeast Petersburg, seizing rifle pits, artillery, and several hundred prisoners. General Beauregard fielded a meager force to defend the town. Had Smith marched straight into the defenseless city, he might have scored a radical breakthrough and altered the war’s course. Grant always believed that with such a move, “Lee would have at once been obliged to abandon Richmond.”