In 1942, J.D. Salinger started dating Oona O'Neill, daughter of the playwright Eugene O'Neill. He called her often and wrote her long letters. Their relationship ended when Oona began seeing Charlie Chaplin, whom she eventually married.

Salinger then began submitting short stories to The New Yorker. Seven of Salinger's stories were rejected by the magazine that year, including "I Went to School with Adolf Hitler".

In December 1941, however, the publication accepted "Slight Rebellion off Madison", a Manhattan-set story about a disaffected teenager named Holden Caulfield with "pre-war jitters".
When Japan carried out the attack on Pearl Harbor that month, the story was rendered "unpublishable"; it did not appear in the magazine until 1946.

In the spring of 1942, several months after the United States entered World War II, Salinger was drafted into the army, wherein he saw combat with the 12th Infantry Regiment, 4th Infantry Division. He was active at Utah Beach on D-Day, in the Battle of the Bulge, and the Battle of Hürtgen Forest.

During the campaign from Normandy into Germany, Salinger arranged to meet with Ernest Hemingway, a writer who had influenced him and was then working as a war correspondent in Paris. Salinger was impressed with Hemingway's friendliness and modesty, finding him more "soft" than his gruff public persona. Hemingway was impressed by Salinger's writing.

Salinger was assigned to a counter-intelligence division, for which he used his proficiency in French and German to interrogate prisoners of war.

In April 1945 he entered a liberated concentration camp, probably one of Dachau's sub-camps. Salinger earned the rank of Staff Sergeant and served in five campaigns.

Salinger's experiences in the war affected him emotionally. He was hospitalized for a few weeks for combat stress reaction after Germany was defeated, and he later told his daughter: "You never really get the smell of burning flesh out of your nose entirely, no matter how long you live."

Salinger continued to write while serving in the army. He also continued to submit stories to The New Yorker, but with little success; it rejected all of his submissions from 1944 to 1946, a group of 15 poems in 1945 alone.

After Germany's defeat, Salinger signed up for a six-month period of "Denazification" duty in Germany for the Counterintelligence Corps. He lived in Weissenburg and, soon after, married Sylvia Welter. He brought her to the United States in April 1946, but the marriage fell apart after eight months and Sylvia returned to Germany.

As the notoriety of The Catcher in the Rye grew, Salinger gradually withdrew from public view.

In 1953, he moved from an apartment at 300 East 57th Street, New York, to Cornish, New Hampshire.

Early in his time at Cornish he was relatively sociable, particularly with students at Windsor High School. Salinger invited them to his house frequently to play records and talk about problems at school. Salinger then cut off all contact with the high schoolers without explanation. He was also seen less frequently around town, meeting only one close friend—jurist Learned Hand—with any regularity. He also began to publish with less frequency.

In 1972, Salinger's daughter Margaret was with him when he received a letter from Sylvia. He looked at the envelope, and without reading it, tore it apart. It was the first time he had heard from her since the breakup, but as Margaret put it, "when he was finished with a person, he was through with them."