The Admiralties sit only two degrees south of the equator, 200 miles northeast of New Guinea's mainland and 360 miles west of Rabaul, the great Japanese bastion that had anchored the enemy's southwestern Pacific defense since 1942. By early 1944, Rabaul's fangs had been pulled - its airfields hammered to rubble, its naval support withdrawn to Truk - but 100,000 Japanese troops remained there, and MacArthur's planners wanted the fortress permanently sealed. The Admiralties offered the seal. They contained flat land for airstrips and, crucially, Seeadler Harbour, a sheltered anchorage 15 miles long and capable of holding the entire US Seventh Fleet. Lieutenant General George Kenney put the case to MacArthur plainly. Take the islands now with whatever force is available, and Rabaul becomes a prison. MacArthur paced his office while Kenney talked, then stopped. "That will put the cork in the bottle," he said.

The orders went out on 24 February. The 1st Cavalry Division was to land on Los Negros - the third-largest island in the group - on 29 February. To reach the islands in time, the assault force needed high-speed transports. Only three were available. Each could carry 170 men. The remaining troops rode the decks of nine destroyers, which were never designed to land soldiers on hostile beaches. MacArthur himself intended to go - so did Vice Admiral Thomas Kinkaid, commander of Allied Naval Forces - which required pulling the light cruiser USS Phoenix from Brisbane, where over 300 of her crew were ashore on leave. Trucks mounted with bullhorns rolled through the streets broadcasting the recall code word. The Phoenix sailed. The cavalry carried one rifle per man and rations enough for a few days. If the Japanese were really gone, they would occupy the islands and start building. If the Japanese were there, they would withdraw.

On 29 February the force approached Los Negros not at the obvious anchorage but at Hyane Harbour, a small, awkward beach on the island's eastern side. The choice surprised the defenders, which was the point. About 4,000 Japanese troops held the Admiralties - not evacuated, just concealed, waiting for the main assault at the other harbor. Tactical surprise worked, but only briefly. The first wave of 1,026 cavalrymen hit the sand against light resistance that thickened within hours. MacArthur came ashore that afternoon in a Navy officer's slacks and walked the perimeter under sniper fire. By nightfall, the Americans held a foothold and the Japanese were massing for counterattack. The reconnaissance in force had become, in the span of a single afternoon, a full assault.

What MacArthur had hoped to accomplish in days took nearly three months. Air superiority and command of the sea meant the Allies could pour reinforcements into Hyane while the Japanese could not reinforce at all. The cavalry expanded across Los Negros, crossed the narrow strait to Manus Island, and ground through jungle and coral in a series of small, grim engagements. The campaign officially ended on 18 May 1944. Roughly 330 Americans died; Japanese losses exceeded 3,000, many of whom refused surrender. But the strategic prize was exactly what MacArthur had promised. Rabaul became a prison for 100,000 Japanese soldiers who would fight no more battles. In Seeadler Harbour, a fleet anchorage began taking shape that would eventually support the invasions of the Philippines, Okinawa, and - had the war continued - the Japanese home islands themselves.