The southern states threatened to leave the Union because of antislavery talks.
Chat with our AI personalities
Secession.
The North was called the Union because it represented those states that remained in "union" together as the Unites States, and because it was fighting to preserve that union by keeping the rebellious Southern states from leaving it.
The Southern States believed that they had the right to secede because the Union had violated the US Constitution. Because of this the Southern States believed that the Republican dominated Congress and the views of President elect Lincoln would continue to do so, thus the Southern States believed that they had the right to leave the Union.
When Lincoln took office in March 1861 seven southern states had left the Union.
The union of Southern states during the US Civil War was named "The Confederate States of America".