In February, 1862 Ulysses S. Grant took his army along the Tennessee River with a flotilla of gunboats and captured Fort Henry. This broke the communications of the extended Confederate line and Joseph E. Johnston decided to withdraw his main army to Nashville. He left 15,000 men to protect Fort Donelson on the Cumberland River but this was not enough and Grant had no difficulty taking this prize as well. With western Tennessee now secured, Abraham Lincoln was now able to set up a Union government in Nashville by appointing Andrew Johnson as its new governor.
After capturing Fort Donelson Ulysses S. Grant advanced up the Tennessee River and established the headquarters of the Army of Tennessee at Savannah. Grant then arranged for his troops to join with Army of the Ohio, led by General Don Carlos Buell. General Albert S. Johnston decided to defeat Grant before Buell arrived.
On 6th April, Albert S. Johnston and Pierre T. Beauregard and 55,000 members of the Confederate Army attacked Grant's army near Shiloh Church, in Hardin, Tennessee. Taken by surprise, Grant's army suffered heavy losses until the arrival of General Don Carlos Buell and reinforcements the following day.
During the fighting Albert S. Johnston was killed and the new commander, Pierre T. Beauregard, decided to retreat to Corinth, Mississippi. Shiloh was the greatest battle so far of the Civil War. The Union Army suffered 13,000 casualties and the Confederate Army lost 10,000. These figures were questioned by Ulysses S. Grant who claimed that Confederates lost many more than he did.