Grant then ordered an immediate assault on Fort Donelson, which dominated the Cumberland River. Fort Donelson, unlike Fort Henry, had a force equal to Grant's army. Unaware of the garrison's strength, Grant, McClernand, and Smith positioned their divisions around the fort. The next day McClernand and Smith independently launched probing attacks on apparent weak spots but were forced to retreat by the Confederates. On February 14, Foote's gunboats began bombarding the fort, only to be repulsed by its heavy guns. Seizing the initiative, the next day, Pillow fiercely attacked and routed one of Grant's divisions, McClernand's. Union reinforcements arrived, giving Grant a total force of over 40,000 men. Grant was with Foote, four miles away when the Confederates attacked. Hearing the battle noise, Grant rode back and rallied his troop commanders, riding over seven miles of freezing roads and trenches, exchanging reports. When Grant blocked the Nashville Road, the Confederates retreated back into Fort Donelson. On February 16, Foote resumed his bombardment, which signaled a general attack. Confederate generals John B. Floyd and Pillow fled, leaving the fort in command of Simon Bolivar Buckner, who submitted to Grant's demand for "unconditional and immediate surrender".