The correct answer to this open question is the following.
Although there are no options attached we can say the following.
What helped the Union military in being able to defeat the Confederate forces was that the troops of the Confederation, led by General Robert. E. Lee, withdrew from the battleground and march away from the combat, dropping the idea to invade the Union's territory. This represented a victory, a strategic one, for the Union troops.
The Battle of Antietam was fought on September 17, 1862, as part of the hostilities in the Civil War. The scene of the battle was Sharpsburg, Maryland. The main goal of the Confederated troops of the Northern Virginia Army -led by General Robert E. Lee- was to invade the North, but Union General George McClellan, who led the Army of the Potomac, impeded it. Historians say that it was a bloody battle in which both armies lost many men.
The Battle of Antietam/Sharpsburg is different from previous battles in the Civil War because it was the first battle on Union soil.