For extraordinary heroism as an officer of the USS Christabel in conflict with an enemy submarine on 21 May 1918. As a result of the explosion of a depth bomb dropped near the submarine, the Christabel was so badly shaken that a number of depth charges which had been set for firing were thrown about the deck and there was imminent danger that they would explode. Ens. Sullivan immediately fell on the depth charges and succeeded in securing them, thus saving the ship from disaster, which would inevitably have caused great loss of life.