The first Essex-class aircraft carriers were ordered as part of the Navys expansion program of 1938. The decision to construct more ships in the series was made after the Pacific war had broken out. The Essex-class aircraft carriers were significant for several reasons. They were large and fast; had strong air defenses; could carry and launch a large air group rapidly; and, importantly, they were available in substantial numbers. Through a combination of these qualities, they played a pivotal role in the U.S. Navys achievements in the Pacific during the final 2 years of World War II.