Robert Leckie, a local sports journalist, enlists in the United States Marine Corps, promising to write to a female neighbour, Vera. A doctor's son, Eugene Sledge, also wants to enlist, but he has a "heart murmur" as told by his father, and his friend, Sidney, enlists without him. Robert and Sidney are both sent to the island of Guadalcanal, where fierce fighting ensues against the Japanese. An USMC NCO, John Basilone, distinguishes himself by his service at the machine gun, carrying it to wherever the enemy attacks even at the cost of his hand, with which he has to grab the overheated barrel. After the battle is won, the marines are sent for rest and recreation in Australia, where they start sexual relations with local ladies; Basilone, however, is separated from the rest and sent back to the US to promote the sale of US war bonds, having been decorated for his heroism. The marines return to the fray (Leckie briefly leaving them as he develops enuresis from the rainy weather), and are eventually joined by Sledge, whose health has ostensibly improved, and who is in the mortar squad. Sledge meets up with his friend briefly, before the latter returns back to the US. Sledge also eventually befriends his squadmate, nicknamed "Snafu". The marines land on and fight a brutal campaign on the island of Peleliu, where Leckie is wounded and evacuated, and many others perish. Meanwhile, Basilone is tired of living the high life as the poster boy for the war bonds effort and returns to train marines at the bootcamp. There he meets and eventually marries a fellow Italian-American servicewoman (who is a Sargent at the kitchen). Later, his unit ships to Iwo Jima where Basilone is soon shot and killed among many others, in a moment of heroism and in slow motion. Meanwhile, Sledge becomes an expert warrior as his mortar squad now is at the island of Okinawa. There, as mental strain increases and brutality grows, news arrive that "a new type of bomb" was dropped on Japan, and the Japanese soon surrender. Leckie returns home and begins dating Vera, to whom he wrote many letters but never sent any, believing he'd die. Sledge returns home after 6 months more of service and has PTSD, while his friend Sidney adjusts much better, but remains loyal to his friend.
