The battle of Fort Donelson, which began on February 12, took place shortly after the surrender of Fort Henry, Tennessee, on February 6, 1862. Fort Henry had been a key position in the center of a line defending Tennessee, and the capture of the fort now opened the Tennessee River to Union troop and supply movements. About 2,500 of Fort Henry's Confederate defenders escaped before its surrender by marching the 12 miles (19 km) east to Fort Donelson. In the days following the surrender at Fort Henry, Union troops cut the railroad lines south of the fort, restricting the Confederates' lateral mobility to move reinforcements into the area to defend against the larger Union forces.