The Civil War battle that caused the most casualties was the Battle of Gettysburg. It resulted in the highest number of casualties in the entire conflict. The Battle of Gettysburg took place from July 1 to July 3, 1863, and is often considered a turning point in the war.
Here are the details:
Battle of Gettysburg: Estimated 51,000 casualties (combined Union and Confederate soldiers)
Battle of Vicksburg: Approximately 37,000 casualties
Battle of Shiloh: Around 23,000 casualties
Battle of Fort Sumter: Few casualties, primarily the initial shots of the war, resulting in minimal loss of life.
Thus, the correct answer is the Battle of Gettysburg.