By the spring of 1865 the Confederacy was a country running out of country. The armies that had carried the rebellion through four years of war were being chased down and cornered, in Virginia and the Carolinas, and the South was shrinking to the few places its soldiers still held. One of those places was Mobile, Alabama, on the Gulf coast. The city itself had been sealed off the year before, when a Union fleet fought its way past the forts at the mouth of Mobile Bay and bottled up the harbor. But the city had not fallen. Behind a ring of earthworks across the bay, the last working Confederate port on the Gulf was still flying its flag.

The Confederacy had been founded, four years and a few weeks earlier, to keep four million Black people in slavery, and the war had become the engine that was tearing slavery apart. Now the engine was running down to its last turns. In January 1865 the general-in-chief of all United States armies, Lieutenant General Ulysses S. Grant, ordered an offensive against the Gulf coast interior: take Mobile, then drive inland and wreck what remained of Confederate Alabama. The man given the job was Major General Edward R. S. Canby (North), commander of the Military Division of West Mississippi, a careful, methodical officer not given to gambling with his men.

Mobile sat on the west side of its bay, but its defenders had built the strongest works on the east side, on the high ground above the rivers that drained into the bay from the north. Two earthworks anchored that line: Spanish Fort near the water, and Fort Blakeley about six miles above it. Canby chose to come at the city the long way, up the eastern shore, taking the forts one at a time rather than throwing his men straight at Mobile. In late March 1865 he ferried his army across to the eastern shore and pushed north, two corps strong, his roughly 45,000 men far outnumbering anything the Confederates could put in front of them.

Among those men were some 5,000 soldiers of the United States Colored Troops, the regiments of Black soldiers, many of them men who had been enslaved not long before, now marching in Union blue. They had marched up from Pensacola, in Florida, with the column of Major General Frederick Steele (North), who set out on March 20 and reached the ground in front of the forts on April 1, with the Black soldiers leading the way. It was one of the largest gatherings of Black troops in any single battle of the war, and the ground they were marching toward was Alabama, deep in the country that had fought to keep them in chains.

Canby went at Spanish Fort first. For nearly two weeks his men dug their way forward in the classic style of a siege, pushing trenches and gun batteries closer night after night while their artillery hammered the works. The fort was held by Brigadier General Randall L. Gibson (South), who put up a stubborn defense against odds of something like ten to one. On the night of April 8, with the Union lines pressing in on him, Gibson pulled most of his garrison out along a narrow causeway through the marsh and got them away, sending roughly a thousand of his men north to reinforce the one fort still standing. That fort was Blakeley. With Spanish Fort emptied, Canby turned his whole weight on it the very next day.