The commanding General of the Union Army Ulysses S. Grant had won promotion, and made himself a reputation, by the capture of Fort Donelson in February: seven weeks later he almost lost it at Shiloh. The Confederate onslaught from Albert Johnston’s army at dawn on 6 April was wholly unexpected, and ferocious, if uncoordinated. The Union soldiers were steadily driven back towards the Tennessee River. Grant was 6 miles (10 km) away, convalescing from a fall from his horse, and while he travelled to the battlefield by steamboat a division of the Union army desperately fought off repeated Confederate attacks at the ‘Hornet’s Nest’, and Johnston was slain. This enabled Grant to marshal the rest of his forces beside the Tennessee, and after fighting broke off at nightfall, Union reinforcements led by Carlos Buell began to arrive from across the river. In a second day of fierce conflict, the advantage swung repeatedly, until, running low on ammunition, Johnston’ replacement Beauregard ordered a Confederate withdrawal.