By mid-1943 American planners began thinking in terms of recapturing the Philippines, but the presence of the Japanese in the Bismarck Archipelago prevented such an undertaking. To breach this barrier necessitated the opening of the Vitiaz and Dampier Straits, separating New Britain and New Guinea, and the occupying of western New Britain by American forces. The control of the Straits' would give General MacArthur's forces an opening into the Japanese bases along the New Guinea coast and a secure approach route to Philippines.

The 1st Marine Division, commanded by Major General William H. Rupertus, was given the task of establishing the American presence in western New Britain. The landings were slated to take place at Cape Gloucester on 26 December 1943. Eleven days prior to the Marine invasion, the Army's 112th Cavalry made an assault on the Arawe Peninsula to the southeast of Cape Gloucester for the primary purpose of distracting the Japanese from the main Marine thrust.

The convoy carrying the 1st Marine Division arrived at its destination early in the morning of the 26th. After a light naval and air bombardment, the Marines embarked on landing craft and headed towards the beaches. Under an aerial smoke screen, which only served to blind and confuse landing craft coxswains, the Marines swarmed ashore at 0748.

Two separate landings were made at beaches about 12 miles apart. The main landing occurred at the Yellow Beaches where Colonel Julian N. Frisbie and his 7th Marines secured the initial beachhead. Units of the 1st Marines, commanded by Colonel William 0. Whaling, landing after the 7th Marines, headed for the Cape Gloucester Airfield. Resistance was relatively light; however, one bitter but brief firefight took place the morning of D-Day. For a time 3/1 was held up by a well entrenched force of Japanese. Tanks finally had to be brought up to break through enemy defenses. The engagement cost the Japanese 25 dead and the Marines 7 dead and 7 wounded. This was the sharpest struggle of the day. That night however, an enemy battalion unsuccessfully attempted to break through the section of the perimeter held by 2/7 and the resulting battle was as fierce as any fought during the campaign.

While the main assault force was hitting the beach, Colonel James M. Masters, 2/1 landed unopposed on Green Beach. The reinforced battalion's objective was to seal off the main coastal trail which led south from Cape Gloucester, thus preventing the escape of retreating enemy units.

Although the invading force met only light resistance, the Americans had to face another obstacle--the dense, tropical rain forest that covered most of New Britain. In many areas it was almost impenetrable. The jungle and swamp combined to hamper the maneuverability of the opposing forces. Nonetheless, the Marines on Yellow Beach resolutely pushed toward their objectives. By 29 December, elements of the 1st Marines reached the airfield on Cape Gloucester. The Japanese withdrew to higher ground as the Marines moved in to occupy the airfield. The next morning the enemy launched a counterattack, but by noon they had been repulsed and the airfield was declared secure. Thus, one of the main objectives of the New Britain operation had been secured and the Marines now began to move inland-in search of the enemy.

After four days of quiet, Lieutenant Colonel Masters, force located at the Green Beach perimeter was attacked by a number of retreating Japanese. The result was a night-long battle which ended when most of the attackers were killed. This engagement ended all serious opposition in the area, and after 10 days of patrolling 2/1 linked up with the rest of the division at the airdrome.

On New Years, day 1944, Brigadier General Lemuel C. Shepherd, the Assistant Division Commander, launched a drive south toward Borgen Bay. One of the most difficult battles of the entire New Britain campaign occurred during this operation. On 8 January 1944, Lieutenant Colonel Lewis W. Walt's 3/5 came across a considerable number of enemy bunkers on Aogiri Ridge. The Japanese were well concealed in the very dense jungle and, as a result the American attack became stalled the first day. The following day, through the leadership and courage of Lieutenant Colonel Walt, the tide of battle turned. While under fire he personally took command of a 37mm gun and with volunteers manhandled the gun up the slope and into position to sweep the ridge.

By nightfall the Marines had forced their way to the crest where they dug in and waited for an expected counterattack from the enemy. Early on the morning of the 10th, the Japanese struck with the aim of hurling back the Americans. The enemy made five repeated charges up the slope, but all failed to dislodge 3/5. As dawn approached the battle was over and not a single Japanese of the attacking force remained alive.

The only remaining stronghold in Japanese hands in the area was Hill 660. Lieutenant Colonel Henry W. Buse, commanding 3/7, was given the mission of taking the jungle height. After two days of probing, Buse led his exhausted men to the summit in a sudden rush. Despite heavy enemy fire, the advance this time was not halted and as night fell the objective had been taken. Two days later, 16 January, the Japanese counterattacked. A few enemy soldiers managed to reach the top but were overwhelmed in the hand-to-hand fighting that ensued. The rest were driven back by the tremendous volume of small-arms fire of the defenders. The Americans then pounded the ranks of the enemy with mortars. This ended the assault and annihilated the attackers. This counterattack alone cost the enemy 110 dead. The overall price to the Japanese for Hill 660 was 200 dead and an unknown number wounded. The cost to the Marines was about 50 men killed and wounded.

The capture of Hill 660 and the repulse of the counterattack marked the effective end of the Japanese defense of the Cape Gloucester--Borgen Bay area. In the following weeks, patrols were continually sent out to harass the retreating Japanese who were attempting to reach Rabaul, the huge enemy bastion on the opposite end of the island.

To intercept the withdrawing enemy, an assault was ordered on the Willaumez Peninsula, some 120 miles east of Cape Gloucester. The Marines were directed to land midway up the peninsula and drive toward the airstrip at Talasea. On 6 March 1944, units of the 5th Marines headed for shore. The enemy in this case decided to oppose the attackers on the beach. Overcoming some determined resistance, 1/5 established a beachhead from which the 2d Battalion struck out for the airstrip at Talasea. For the next three days, a series of small actions occurred in the vicinity-after which the area was freed from Japanese control. The following weeks saw the regiment engaged in numerous patrols to cut off enemy units.
A number, of clashes occurred in which 150 prisoners were taken.

With the seizure of Talasea the campaign for western New Britain by the 1st Marine Division ended, although active patrolling continued well into April. On the 28th of that month the Marines were relieved by the Army's 40th Infantry Division.

The 1st Marine Division left New Britain knowing that it had accomplished its assigned mission. Western New Britain with its airfields was in American hands. Moreover, the vital Japanese supply route between Rabaul and New Guinea had been severed. As a result the door to the Philippines was pushed further ajar. Finally, one more link had been added to the American chain that was encircling Rabaul.

The cost to the Marines for the fourth month campaign was 478 killed and 982 wounded.