American Civil War: The Siege of Port Hudson ends in a Union victory and, along with the fall of Vicksburg five days earlier, gives the Union complete control of the Mississippi River.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

The siege of Port Hudson was the final engagement in the Union campaign to recapture the Mississippi River in the American Civil War. While Union General Ulysses Grant was besieging Vicksburg upriver, General Nathaniel Banks was ordered to capture the lower Mississippi Confederate stronghold of Port Hudson, Louisiana, to go to Grant's aid. When his assault failed, Banks settled into a 48-day siege, the longest in US military history up to that point. A second attack also failed, and it was only after the fall of Vicksburg that the Confederate commander, General Franklin Gardner, surrendered the port. The Union gained control of the river and navigation from the Gulf of Mexico through the Deep South and to the river's upper reaches.

The Union was the central government of the United States during the American Civil War. Its civilian and military forces resisted the Confederacy's attempt to secede following the election of Abraham Lincoln as president of the United States. Lincoln's administration asserted the permanency of the federal government and the continuity of the United States Constitution.

The siege of Vicksburg was the final major military action in the Vicksburg campaign of the American Civil War. In a series of maneuvers, Union Major General Ulysses S. Grant and his Army of the Tennessee crossed the Mississippi River and drove the Confederate Army of Mississippi, led by Lieutenant General John C. Pemberton, into the defensive lines surrounding the fortress city of Vicksburg, Mississippi, leading to the successful siege and Confederate surrender.