OTD in 1864: 100,000 men vanished overnight, and the greatest general of the age had no idea where they went. This might be the most underrated move of the Civil War.
Context: Grant had just spent ten days locked in trench warfare at Cold Harbor, Virginia, after a frontal assault on June 3 that cost him thousands of men in under an hour. He admitted it was the worst mistake of his career. The armies were so close that soldiers could not lift their heads above the dirt in daylight.
Everyone, including Lee, expected Grant to do what every Union commander before him did after a bloody repulse: retreat north and regroup.
Instead, on the night of June 12, Grant did something audacious. He pulled the entire Army of the Potomac out of trenches that were in some places only yards from Confederate lines. No bugles, no fires, wheels muffled. By morning the Union trenches were empty and Lee's scouts found nothing but abandoned earthworks.
The army marched south, away from Richmond, which made no sense to Confederate observers. Then Union engineers did something almost nobody thought possible: they threw a pontoon bridge across the James River, roughly 2,100 feet of it, over water up to 85 feet deep with a four-foot tidal swing. They built it in about eight hours. It was one of the longest floating bridges in military history.
For three full days Lee was effectively blind, unsure whether Grant was north or south of the James. By the time the picture cleared, Grant's army was across the river attacking Petersburg, the rail hub that fed Richmond.
The siege that followed lasted nine months and ended with Lee's surrender at Appomattox.
Everyone remembers Cold Harbor as Grant's worst day. Almost nobody remembers that one week later he pulled off the maneuver that won the war.