After the Confederate artillery fired on Fort Sumter in April 1861.
Lincoln regarded this as an act of war, and called for 75,000 volunteers for a new Union army (which Sherman said would be about as much use as a water-pistol).
The South saw this as a provocation, and four more states joined the Confederacy.
The war was on.
The Union Army
Every battle fought by the Army of Northern Virginia was against the Union army.
The Northern, or Union Army
The Union Army was the Northern army during the American Civil War. The Union Army fought and defeated the Confederate States (Southern) Army in 1865.
The Union army (or armies)
Confederate army was the name. They were also called Rebels too.
Billy Yank
the Union
The Union Army of the Potomac, and the Confederate Army of Northern Virginia.
because they did
'the union' and the southern was 'the rebels'
The Southern army is known as the "Confederate" army. The Northern army is known as the "Union" army.