Driven by monsoon winds, the rain that screened the attack on Conoley's 2d Battalion, 7th Marines, drenched the entire island and everyone on it. At the front, the deluge flooded foxholes, and conditions were only marginally better at the rear, where some men slept in jungle hammocks slung between two trees. A Marine entered his hammock through an opening in a mosquito net, lay down on a length of rubberized cloth, and zipped the net shut. Above him, also enclosed in the netting, stretched a rubberized cover designed to shelter him from rain. Unfortunately, a gale as fierce as the one that began blowing on the night of D-Day set the cover to flapping like a loose sail and drove the rain inside the hammock. In the darkness, a gust of wind might uproot a tree, weakened by flooding or the effect of the preparatory bombardment, and send it crashing down. A falling tree toppled onto a hammock occupied by one of the Marines, who would have drowned if someone had not slashed through the covering with a knife and set him free.

The rain, said Lieutenant Colonel Lewis J. Fields, a battalion commander in the 11th Marines, resembled "a waterfall pouring down on you, and it goes on and on." The first deluge lasted five days, and recurring storms persisted for another two weeks. Wet uniforms never really dried, and the men suffered continually from fungus infections, the so-called jungle rot, which readily developed into open sores. Mosquito-borne malaria threatened the health of the Marines, who also had to contend with other insects—"little black ants, little red ants, big red ants," on an island where "even the caterpillars bite." The Japanese may have suffered even more because of shortages of medicine and difficulty in distributing what was available, but this was scant consolation to Marines beset by discomfort and disease. By the end of January 1944, disease or non-battle injuries forced the evacuation of more than a thousand Marines; more than one in ten had already returned to duty on New Britain.

The island's swamps and jungles would have been ordeal enough without the wind, rain, and disease. At times, the embattled Marines could see no more than a few feet ahead of them. Movement verged on the impossible, especially where the rains had flooded the land or turned the volcanic soil into slippery mud. No wonder that the Assistant Division Commander, Brigadier General Lemuel C. Shepherd, Jr., compared the New Britain campaign to "Grant's fight through the Wilderness in the Civil War."