The 7th Infantry Division began by capturing the small islands labeled Carlos, Carter, Cecil, and Carlson on 31 January, which were used as artillery bases for the next day's assault. Kwajalein Island is 2.5 mi (4.0 km) long but only 880 yd (800 m) wide. There was therefore no possibility of defence in depth, so the Japanese planned to counter-attack the landing beaches. They had not realized until the battle of Tarawa that American amphibious vehicles could cross coral reefs and so land on the lagoon side of an atoll; accordingly the strongest defences on Kwajalein faced the ocean. The bombardment by battleships, B-24 bombers from Apamama and artillery on Carlson was devastating. The U.S. Army history of the battle quotes a participant as saying that "the entire island looked as if it had been picked up 20,000 feet and then dropped." By the time the 7th Division landed on Kwajalein Island on 1 February, there was little resistance; by night the Americans estimated that only 1,500 of the original 5,000 defenders were still alive.
On the north side of the atoll, the 4th Marine Division followed the same plan, first capturing islets Ivan, Jacob, Albert, Allen, and Abraham on 31 January, and landing on Roi-Namur on 1 February. The airfield on Roi (the eastern half), was captured quickly, and Namur (the western half), fell the next day. The worst setback came when a Marine demolition team threw a satchel charge of high explosives into a Japanese bunker which turned out to be a torpedo warhead magazine. The resulting explosion killed twenty Marines and wounded dozens more. Only 51 of the original 3,500 Japanese defenders of Roi-Namur survived to be captured.