By early March 1942, the Japanese had turned Rabaul on the island of New Britain and Simpson Harbor whom it overlooked, into their primary forward base and logistics hub in the South Pacific. In less than 45 days, Rabaul’s naval and air capacity rivaled any comparable American base in the Pacific, and was the lynchpin in the Japanese perimeter defense for the next two years. Rabaul was also the spring board for further operations to the south and west.
On 8 March 1942, Japanese forces that overran Rabaul landed at Salamaua–Lae on the eastern coast of Papua New Guinea. New Guinea is the second largest island on the planet, but in 1942, control of the island required the Australian colonial capital of Poet Moresby on the southeastern coast. The speed at which the Japanese moved from Rabaul to the landings at Salamaua–Lae took the Allies completely by surprise. After the collapse of ABDACOM (the now nearly unknown seminal event of the first six months of 1942 in the Pacific after Pearl Harbor), FDR personally ordered MacArthur out of the Philippines to take command of American and Australian forces in the Southwest Pacific. He hadn’t left yet, but the reports of Japanese landings on New Guinea convinced him he was way behind the power curve and needed to leave, no matter what it looked like to his troops on Bataan and Corregidor.
The landings at Salamaua–Lae were not contested by the minuscule Australian garrison, which destroyed its equipment and retreated into the interior. However, the American carriers Yorktown and Lexington, on their way to raid Rabaul, diverted and with American and Australian land based bombers, attacked the anchored invasion force on 10 March. They sank three transports and damaged several others. But by then the Japanese were already established.
MacArthur would later regret handing Salamaua–Lae to the Japanese with so little resistance. It was the only suitable, and obvious, place to land on Northeastern New Guinea, and its capture (along with Timor far off the west coast) rendered all but the southeast corner of the island untenable to the Allies. But the Allies continued to underestimate Japanese operational agility. It would take 18 mos of bloody jungle fighting to regain the area from the Japanese.
More immediately, the airfields at Salamaua–Lae provided a perfect forward base for air domination over the real prize in New Guinea: Port Moresby on the Coral Sea.