On New Britain, the 1st Marine Division fought weather and terrain, along with a determined Japanese enemy. Rains brought by seasonal monsoons seemed to fall with the velocity of a fire hose, soaking everyone, sending streams from their banks, and turning trails into quagmire. The terrain of the volcanic island varied from coastal plain to mountains that rose as high as 7,000 feet above sea level. A variety of forest covered the island, punctuated by patches of grassland, a few large coconut plantations, and garden plots near the scattered villages.

Much of the fighting, especially during the early days, raged in swamp forest, sometimes erroneously described as damp flat. The swamp forest consisted of scattered trees growing as high as a hundred feet from a plain that remained flooded throughout the rainy season, if not for the entire year. Tangled roots buttressed the towering trees, but could not anchor them against gale-force winds, while vines and undergrowth reduced visibility on the flooded surface to a few yards.

No less formidable was the second kind of vegetation, the mangrove forest, where massive trees grew from brackish water deposited at high tide. Mangrove trees varied in height from 20 to 60 feet, with a visible tangle of thick roots deploying as high as ten feet up the trunk and holding the tree solidly in place. Beneath the mangrove canopy, the maze of roots, wandering streams, and standing water impeded movement. Visibility did not exceed 15 yards.

Both swamp forest and mangrove forest grew at sea level. A third form of vegetation, the true tropical rain forest, flourished at higher altitude. Different varieties of trees formed an impenetrable double canopy overhead, but the surface itself remained generally open, except for low-growing ferns or shrubs, an occasional thicket of bamboo or rattan, and tangles of vines. Although a Marine walking beneath the canopy could see a standing man as far as 50 yards away, a prone rifleman might remain invisible at a distance of just ten yards.

Only one of the three remaining kinds of vegetation seriously impeded military action. Second-growth forest, which often took over abandoned garden tracts, forced patrolling Marines to hack paths through the small trees, brush, and vines. Grasslands posed a lesser problem; though the vegetation grew tall enough to conceal the Japanese defenders, it provided comparatively easy going for the Marines, unless the grass turned out to be wild sugar cane, with thick stalks that grew to a height of 15 feet. Cultivated tracts, whether coconut plantations or gardens, posed few obstacles to vision or movement.