Under the command of Major General Alexander Patch, the Americal Division was the first US Army unit to be sent to Guadalcanal, where it relieved the exhausted US 1st Marine Division, and carried the brunt of the fighting on the island from that time on.
Largely because of transport constraints, the Americal arrived piecemeal, and was fed into combat alongside the battle-hardened marines, and thus, in contrast to several other US Army divisions in the Pacific War, was able to learn the practical art of war against the Japanese without suffering as many casualties as might otherwise have eventuated. Despite its ad-hoc formation, the Americal Division fought well at Guadalcanal, the 164th Regiment taking on a key role in the defeat of the major Japanese offensive in October 1942, and historians describe it as the most effective of all the US Army divisions in that conflict.
Later in World War II the Americal Division (alongside the 37th Division, a Marine defense battalion, and supporting units) took up positions on the newly invaded island of Bougainville, and warned by intelligence of the storm to come, utterly defeated a massive and sustained Japanese counter-attack, which began on 7 March 1943. Despite ample warning and thorough defensive preperations, the battle soon degenerated into a bitter, close-quarters infantry affair, with artillery restricted by the need to avoid friendly troops and tanks unable to reach the scene. The 37th and Americal Divisions stood firm, and by March 25, the Japanese were forced to retreat. It was the last Japanese ground offensive in the South Pacific.