The landings at Cape Merkus in mid-December caused Matsuda to shift his troops to meet the threat, but this redeployment did not account for the lack of resistance at the Yellow Beaches. The Japanese general, familiar with the terrain of western New Britain, did not believe that the Americans would storm these strips of sand extending only a few yards inland and backed by swamp. Matsuda might have thought differently had he seen the American maps, which labeled the area beyond the beaches as "damp flat," even though aerial photographs taken after preliminary air strikes had revealed no shadow within the bomb craters, evidence of a water level high enough to fill these depressions to the brim. Since the airfields were the obvious prize, Matsuda did not believe that the Marines would plunge into the muck and risk becoming bogged down short of their goal.

Besides forfeiting the immediate advantage of opposing the assault force at the water's edge, Matsuda's troops suffered the long-term, indirect effects of the erosion of Japanese fortunes that began at Guadalcanal and on New Guinea and continued at New Georgia and Bougainville. The Allies, in addition, dominated the skies over New Britain, blunting the air attacks on the Cape Merkus beachhead and bombing almost at will throughout the island. Although air strikes caused little measurable damage, save at Rabaul, they demoralized the defenders, who already suffered shortages of supplies and medicine because of air and submarine attacks on seagoing convoys and coastal shipping. An inadequate network of primitive trails, which tended to hug the coastline, increased Matsuda's dependence on barges, but this traffic, hampered by the American capture of Cape Merkus, proved vulnerable to aircraft and later to torpedo craft and improvised gunboats.

The two battalions that landed on the Yellow Beaches—Weber's on the left and Williams's on the right—crossed the sands in a few strides, and plunged through a wall of undergrowth into the damp flat, where a Marine might be slogging through knee-deep mud, step into a hole, and end up, as one on them said, "damp up to your neck." A counterattack delivered as the assault waves wallowed through the damp flat might have inflicted severe casualties, but Matsuda lacked the vehicles or roads to shift his troops in time to exploit the terrain. Although immobile on the ground, the Japanese retaliated by air. American radar detected a flight of enemy aircraft approaching from Rabaul; Army Air Forces P-38s intercepted, but a few Japanese bombers evaded the fighters, sank the destroyer Brownson with two direct hits, and damaged another.

The first enemy bombers arrived as a squadron of Army B-25s flew over the LSTs [Landing Ships, Tank] en route to attack targets at Borgen Bay south of the Yellow Beaches. Gunners on board the ships opened fire at the aircraft milling overhead, mistaking friend for foe, downing two American bombers, and damaging two others. The survivors, shaken by the experience, dropped their bombs too soon, hitting the artillery positions of the 11th Marines at the left flank of Yellow Beach 1, killing one and wounding 14 others. A battalion commander in the artillery regiment recalled "trying to dig a hole with my nose," as the bombs exploded, "trying to get down into the ground just a little bit further."

By the time of the air action on the afternoon of D-Day, the 1st Marine Division had already established a beachhead. The assault battalions of the 7th Marines initially pushed ahead, capturing Target Hill on the left flank, and then paused to await reinforcements. During the day, two more battalions arrived. The 3d Battalion, 1st Marines—designated Landing Team 31 and led by Lieutenant Colonel Joseph F. Hankins, a Reserve officer who also was a crack shooter—came ashore at 0815 on Yellow Beach 1, passed through the 3d Battalion, 7th Marines, and veered to the northwest to lead the way toward the airfields. By 0845, the 2d Battalion, 7th Marines, under Lieutenant Colonel Odell M. Conoley, landed and began wading through the damp flat to take its place between the regiment's 1st and 3d Battalions as the beachhead expanded. The next infantry unit, the 1st Battalion, 1st Marines, reached Yellow Beach 1 at 1300 to join that regiment's 3d Battalion, commanded by Hankins, in advancing on the airfields. The 11th Marines, despite the accidental bombing, set up its artillery, an operation in which the amphibian tractor played a vital part. Some of the tractors brought lightweight 75mm howitzers from the LSTs directly to the battery firing positions; others broke trail through the undergrowth for tractors pulling the heavier 105mm weapons.

Meanwhile, Army trucks loaded with supplies rolled ashore from the LSTs. Logistics plans called for these vehicles to move forward and function as mobile supply dumps, but the damp flat proved impassable by wheeled vehicles, and the drivers tended to abandon the trucks to avoid being left behind when the shipping moved out, hurried along by the threat from Japanese bombers. Ultimately, Marines had to build roads, corduroying them with logs when necessary, or shift the cargo to amphibian tractors. Despite careful planning and hard work on D-Day, the convoy sailed with about 100 tons of supplies still on board.

While reinforcements and cargo crossed the beach, the Marines advancing inland encountered the first serious Japanese resistance. Shortly after 1000 on 26 December, Hankins's 3d Battalion, 1st Marines, pushed ahead, advancing in a column of companies because a swamp on the left narrowed the frontage. Fire from camouflaged bunkers killed Captain Joseph A. Terzi, commander of Company K, posthumously awarded the Navy Cross for heroism while leading the attack, and his executive officer, Captain Philip A. Wilheit. The sturdy bunkers proved impervious to bazooka rockets, which failed to detonate in the soft earth covering the structures, and to fire from 37mm guns, which could not penetrate the logs protecting the occupants. An Alligator that had delivered supplies for Company K tried to crush one of the bunkers but became wedged between two trees. Japanese riflemen burst from cover and killed the tractor's two machine gunners, neither of them protected by armor, before the driver could break free. Again lunging ahead, the tractor caved in one bunker, silencing its fire and enabling Marine riflemen to isolate three others and destroy them in succession, killing 25 Japanese. A platoon of M4 Sherman tanks joined the company in time to lead the advance beyond this first strongpoint.

Japanese service troops—especially the men of the 1st Shipping Engineers and the 1st Debarkation Unit—provided most of the initial opposition, but Matsuda had alerted his nearby infantry units to converge on the beachhead. One enemy battalion, under Major Shinichi Takabe, moved into position late on the afternoon of D-Day, opposite Conoley's 2d Battalion, 7th Marines, which clung to a crescent-shaped position, both of its flanks sharply refused and resting on the marshland to the rear. After sunset, the darkness beneath the forest canopy became absolute, pierced only by muzzle flashes as the intensity of the firing increased.

The Japanese clearly were preparing to counterattack. Conoley's battalion had a dwindling supply of ammunition, but amphibian tractors could not begin making supply runs until it became light enough for the drivers to avoid tree roots and fallen trunks as they navigated the damp flat. To aid the battalion in the dangerous period before the skies grew pale, Lieutenant Colonel Lewis B. Puller, the executive officer of the 7th Marines, organized the men of the regimental Headquarters and Service Company into carrying parties to load themselves down with ammunition and wade through the dangerous swamp. One misstep, and a Marine burdened with bandoliers of rifle ammunition or containers of mortar shells could stumble and drown. When Colonel Frisbie, the regimental commander, decided to reinforce Conoley's Marines with Battery D, 1st Special Weapons Battalion, Puller had the men leave their 37mm guns behind and carry ammunition instead. A guide from Conoley's headquarters met the column that Puller had pressed into service and began leading them forward, when a blinding downpour, driven by a monsoon gale, obscured landmarks and forced the heavily laden Marines to wade blindly onward, each man clinging to the belt of the one ahead of him. Not until 0805, some twelve hours after the column started off, did the men reach their goal, put down their loads, and take up their rifles.

Conoley's Marines had in the meantime been fighting for their lives since the storm first struck. A curtain of rain prevented mortar crews from seeing their aiming stakes, indeed, the battalion commander described the men as firing "by guess and by God." Mud got on the small-arms ammunition, at times jamming rifles and machine guns. Although forced to abandon water-filled foxholes, the defenders hung on. With the coming of dawn, Takabe's soldiers gravitated toward the right flank of Conoley's unit, perhaps in a conscious effort to outflank the position, or possibly forced in that direction by the fury of the battalion's defensive fire. An envelopment was in the making when Battery D arrived and moved into the threatened area, forcing the Japanese to break off the action and regroup.