What was the longest single battle of the American Civil War?

The Battle of Port Hudson, Louisiana (May 22 – July 9, 1863), was the final engagement in the Union campaign to recapture the Mississippi River in the American Civil War. When his assault failed, General Nathaniel Banks settled into a 48-day siege, the longest in US military history up to that point. The battle started when Confederate batteries fired down onto Union gunboats on the Mississippi.

The Battle of Port Hudson involved approximately 7,500 Confederates who resisted some 40,000 Union soldiers for almost two months during 1863. With the Union's capture of Port Hudson and the Confederates' surrender at Vicksburg, it signaled the end. White southerners lost faith in the Confederate Army's ability to win the war. The capital of the Confederate States of America also fell under Union control. The Confederacy could no longer get supplies from western states and Confederate armies in northern states could no longer reach the southern states.

The Battle of Port Hudson and subsequent surrender of the Confederate garrison was an under-appreciated turning point of the Civil War. Not only did the bastion’s fall eliminate the Confederacy’s control over the flow of naval and commercial traffic on the Mississippi River, the fight for this strategic position brought about needed transformations for the overall conduct of the war by both sides.