Answer:
the Battle of Fort Sumter in 1861
Explanation:
In most history books, you will find that The Battle of Fort Sumter (April 12–13, 1861) is considered to be what started the American Civil War. It constituted of the bombardment of Fort Sumter near Charleston, South Carolina by the Confederate States Army. Obviously, this caused a return gunfire and eventual surrender by the United States Army. It's constantly mentioned that this was the excuse that Lincoln needed to put an end to the rebellion.