The Battle of Alcatraz, which lasted from May 2 to 4, 1946, was the result of an unsuccessful escape attempt at Alcatraz Federal Penitentiary. Two corrections officers—William A. Miller and Harold Stites—were killed along with three of the inmates. Eleven corrections officers and one uninvolved convict were also injured. Two of the surviving convicts were later executed for their roles.

On May 2, 1946, while most convicts and corrections officers were in outside workshops, Coy was in the main cellhouse sweeping the floor around C Block when kitchen orderly Marvin Hubbard called on officer William Miller to let him in as he had just finished cleaning the kitchen. As Miller was frisking Hubbard for any stolen articles, Coy attacked him from behind, and the two men overpowered the officer. They then released Joseph Cretzer and Clarence Carnes from their cells.

The cellhouse had an elevated gun gallery that was regularly patrolled by an armed officer. The officer, Burt Burch, had a set routine, and the convicts had attacked Miller while Burch was away. Coy, as a cellhouse orderly, had over the years spotted a flaw in the bars protecting the gun gallery, which allowed them to be widened using a bar-spreading device consisting of a nut and bolt with client metal sleeve that moved when the nut was turned by a small wrench. Coy thus managed to spread the bars and squeeze through the widened gap (Coy starved himself in order to fit through the space between the widened bars, which was still relatively narrow) into the temporarily vacant gallery and to overpower and bind Halsey on his return. Coy kept the Springfield rifle in the gallery and lowered an M1911 pistol, keys, a number of clubs, and gas grenades to his accomplices below.

Continuing along the gun gallery, Coy then entered D Block, which was separated from the main cellhouse by a concrete wall and was used for prisoners kept in isolation. There, he used the rifle to force officer Cecil Corwin to open the door to the main cellhouse and let the others in. They then released about a dozen convicts including Sam Shockley and Miran Thompson. Shockley and Thompson joined Coy, Carnes, Hubbard, and Cretzer in the main cellhouse. The other prisoners returned to their cells. Miller and Corwin were placed in a cell in C Block.

The escapees now needed to secure the key to the yard door of the prison from which they expected to make their way to the island's dock to seize the prison's launch. The boat docked daily between 14:00 and 14:30. The plan was to use the hostage officers as cover as the prisoners made their way to the dock, then San Francisco and freedom.

Miller had held on to the yard door key (against regulations), so that he could let out kitchen staff without having to disturb the gallery officer at lunch. Although they eventually found the key by searching the captive officers and the cell in which the prisoners had placed them, the door would not open because the lock had jammed as the prisoners had tried several other keys while searching for the correct one. The escape attempt was thus inadvertently foiled from the outset as the prisoners were trapped in the cellhouse.

Meanwhile, additional officers who entered the cellhouse as part of their routine were seized along with others sent to investigate when the former officer failed to report in. The prisoners were soon holding nine officers in two separate cells, but with nowhere to go, despair set in among the would-be escapees.

Having failed on their initial plan, the prisoners decided to[3] shoot it out. At 14:30, Coy took the rifle and fired at the officers in some neighboring watchtowers, wounding one of them. Associate warden Ed Miller went to the cellhouse to investigate, armed with a gas billy club. He came across Coy, who shot at him. Miller retreated. By now, the alarm had been raised.

Their plan having failed, Shockley and Thompson urged Cretzer, who had one of the guns, to kill the hostages in case they testified against them. Cretzer opened fire on the officers, wounding five, three seriously, including Bill Miller, who later died of his wounds. Carnes, Shockley, and Thompson then returned to their cells, but Coy, Hubbard, and Cretzer decided they were not going to surrender.[4] Meanwhile, one of the hostages discreetly wrote down the names of the convicts involved, circling the names of the ringleaders.

At about 18:00, a squad of armed officers entering the gun cage were shot at by the convicts. One officer, Harold Stites, was killed, and four other officers were wounded.[4] Prison officials then cut the electricity and put on hold all further attempts to regain control of the cellhouse until darkness.

Warden James A. Johnston now called upon the expertise of two platoons of Marines under the direction of Generals "Vinegar" Joe Stilwell and Frank Merrill to guard the general population of convicts and to take the cellhouse from the outside.

After night fell, two squads of officers entered the prison to locate and rescue the captive officers. There was a long-standing rule at Alcatraz that no guns were allowed in the cellhouse, and the prison officials did not want more officers injured or killed. The convicts' position on the top of a cell block provided a nearly impregnable firing position as they were out of range of the officers in the gun cages.

At 20:00, unarmed officers entered the cellhouse, covered by armed officers in the two gun galleries overhead. They found the hostages; however, one officer was wounded by a gunshot fired from the roof of one of the cell blocks. They locked the open door to D Block. When the last officer reached safety, the officers opened a massive barrage from machine guns, mortars, and grenades on the prisoners within D Block, where the prison authorities erroneously thought one of the armed convicts was holed up. They eventually figured out that the rebellious prisoners were confined to the main cellhouse and ceased their attack until further tactics were worked out.

The Marines implemented a plan to drive the armed convicts into a corner with tactics they had perfected against entrenched Japanese resistance during the Pacific War. They drilled holes in the prison roof and dropped grenades into areas where they believed the convicts were to force them into a utility corridor where they could be cornered.

On May 3, at about 12:00, the convicts phoned Johnston to try to discuss a deal. Johnston would only accept their surrender. Later that day, a shot was fired at an officer as he checked out C Block's utility corridor. That night, a constant fusillade was fired at the cell block until about 21:00. The following morning, squads of armed officers periodically rushed into the cellhouse firing repeatedly into the narrow corridor. At 09:40 on May 4, they finally entered the corridor and found the bodies of Cretzer, Coy, and Hubbard.