After the Battle of Chickamauga in September 1863, Union Gen. William S. Rosecrans retreated to Federal-occupied Chattanooga, a strategically vital rail center, where Confederate Gen. Braxton Bragg laid siege from Lookout Mountain and Missionary Ridge. Union Gen. Ulysses S. Grant took command in October and began efforts to break the siege, while Bragg detached forces under Gen. James Longstreet to attack Knoxville as a diversion. After Gen. William T. Sherman reinforced Grant in November, Federal operations against the heights helped force Bragg to retreat, and the Union army held Chattanooga for the rest of the war. Grant's plan for driving the Confederate Army of Tennessee from the Chattanooga area depended on Sherman's river crossing. Sherman marched his troops from Bridgeport, Alabama, crossed the Tennessee River at Brown's Ferry, and moved into hidden camps behind the hills north of Chattanooga before recrossing the river to attack the northern end of Missionary Ridge. The operation required coordination, secrecy, and the silent construction of a pontoon bridge. Maj. Henry S. Dean's 22nd Michigan Infantry carried two pontoon bridges from downtown Chattanooga to the crossing point north of the city, moving only at night after setting out on November 20 and concealing the pontoons in a ravine west of the river two days later. Meanwhile, Col. Daniel McCook's brigade assembled 116 pontoon boats at the mouth of North Chattanooga Creek. Before daylight on November 24, boats carrying McCook's soldiers pushed off silently and drifted southward. After landing, a thousand Union soldiers secured the riverbank for the crossing, the 22nd Michigan Infantry rushed the pontoons to the river and laid the bridge, and Sherman's troops began crossing just after dawn. The Confederates were taken entirely by surprise, and by noon on November 24 Sherman's men were across the river and ready to attack the northern end of Missionary Ridge the next day, although the Federal center ultimately broke the Confederate line.