The Union forces were the Fort Butler Garrison: two companies of the 28th Maine Volunteer Infantry and some convalescents from various regiments.

On the night of June 27, Green, within a mile and a half of the fort, began moving troops ahead to attack.

The attack started soon after midnight, and the Confederates quickly surrounded the fort and began passing through the various obstructions.

Those troops attacking along the levee came to a ditch, unknown to them, too wide to cross, that saved the day for the Union garrison.

A Union gunboat, USS Princess Royal, came to the garrison's aid also and began shelling the attackers.