After the invasion of Tarawa in November, the next step for Adm Nimitz’ Central Pacific Campaign were the Marshall Islands, specifically the Kwajalein Atoll.
On 29 Jan 1944, Nimitz’ Bombardment Group centered around 6 fleet carriers, 6 Escort carriers, and eight battleships shelled, staffed and bombed the islands of the atoll. On the 31st, the US 7th Infantry and 4th Marine Divisions assaulted the islands of Kwajalein and Roi-Namur. The Soldiers and Marines never encountered more than 300 dazed survivors at a time. Within a week, the Kwajalein atoll was secured.
Although there was hard fighting at times, Operation Flintlock was successful because it was characteristic of the changing nature of the war in the Pacific, and the Japanese slowness to adapt. The Japanese fortified the outer ring islands of the atoll, but the Americans broke their code and knew which islands to isolate. Without a challenge from the Japanese Navy (which still had not replaced the losses from the Guadcanal naval battles), the US Navy just sailed around the heavily defended islands and secured the supporting islands. Without a navy to come to their aid, most Japanese were cut off and just left alone by the Americans to starve to death.
Furthermore, the heaviest Japanese defenses were focused on the ocean side of the islands because they believed the Americans did not have the technology to penetrate the lagoon or wherewithal to risk another Betio. They were mistaken. The Americans learned from their mistakes of the last year, and either found solutions or did not repeat them. So if any heavily defended islands had to be attacked, the Allied assaults hit Japanese defenses pointed on the wrong direction. Finally, the pre invasion bombardments were particularly effective. The Japanese weren’t as heavily fortified on the supporting islands. Estimates from both Japanese and American sources say that 50% of the 8000 Japanese defenders on the atoll were killed or wounded before a single soldier or Marine set foot on dry ground.
The Kwajalein and nearby Eniwetok atolls would provide the springboards for the American return to Guam and the Mariana Islands later in the year.