On the morning of July 21, 1944 an advanced party from the 14th Defense Battalion went ashore on Guam from the USS President Monroe (AP-104).  The rest of the battalion came ashore a few hours after the initial assault waves stormed the beach. By night of the first day all of the battalion's anti-aircraft batteries were emplaced ashore with some also providing direct fire support for Marines of the 3rd Marine Regiment. On  July 23, the battalion received orders to occupy Cabras Island in Apra Harbor. From Cabras Island the battalion was able to continue its mission of providing air defense for Orote Field and provide flanking artillery fire for Marines on the Orote Peninsula and in the Northern part of the island.  The battalion's command post moved from Cabras to the Orote Peninsula on August 5. The smokescreen unit that had been attached to the battalion prior to the battle was disbanded in July 1944 because the concept did not prove feasible.   During the battle the battalion served under the command of the 12th Marine Regiment.