After a series of losses early in the war, Union troops tumed the tide by winning
an important battle at
Bull Run.
Fort Sumter.
Richmond.
Antietam.

Antietam.

Explanation:

After winning the Second Battle of Bull Run over the Northern Army, Confederate General Robert E. Lee decided to invade the North and transfer the war from Confederate territory to Union territory. He penetrated with 55,000 troops across the Potomac River into the territory of the northern state of Maryland. To counter it, US President Lincoln sent the Army of the Potomac under the command of George B. McClellan. The battle took place on September 17, 1862 near the creek of Antietam. It was one of the bloodiest battles in all of American history. Although he had a double army of about 87,000 troops, Northern General McClellan sent less than three-quarters of his troops to war. In total, about 23,000 soldiers were killed or disappeared. McClellan failed to destroy the Southern Army, but Lee had to end the invasion of the North and retreat to Virginia.