In the United States Navy, the expeditionary strike group (ESG) is a coordinated group of surface ships, aircraft, submarines, and other naval assets. In contrast to carrier strike groups (CSGs), which emphasize air power and are led by a supercarrier, ESGs are strongly suited for amphibious warfare and are led by an amphibious assault ship (currently of the Wasp or America classes). The ESG concept was introduced in the early 1990s, based on the Naval Expeditionary Task Force. The U.S. Navy fields nine expeditionary strike groups and ten carrier strike groups, in addition to surface action groups.