The Mississippi River.
Abner Doubleday ( who invented baseball's rules ) was stationed in Fort Sumter as a Union officer.
The conflict between the Confederacy and the United States began in the harbor of Charleston, South Carolina. There Southern forces used cannon fire on the US Fort Sumter and forced its commander to surrender. This conflict led to the US Civil War. The Confederacy passed an act in May of 1861, declaring a state of war existed between itself & the United States. Lincoln and others in the North continued to call this a rebellion until July, 1861. This date was noted by the US Supreme Court as the beginning of the war.
It was fought from 1861 ro 1864