It tells the story of Robert Jordan, a young American volunteer attached to a Republican guerrilla unit during the Spanish Civil War.

In 1940, the year the book was published, the United States had not yet entered World War II, which began on September 1, 1939, with Nazi Germany's invasion of Poland.

[2] Ernest Hemingway wrote For Whom the Bell Tolls in 1939 from three locations: Havana, Cuba; Key West, Florida; and Sun Valley, Idaho.

[11] The novel graphically describes the brutality of the war and is told primarily through the thoughts and experiences of the protagonist, Robert Jordan.

Set in the Sierra de Guadarrama mountain range between Madrid and Segovia, the action takes place during four days and three nights.

On his mission, Jordan meets the rebel Anselmo, the "old man", who brings him to the hidden guerrilla camp in the Sierra de Guadarrama mountains between Madrid and Segovia.

In the camp, Jordan encounters María, a young Spanish woman whose life has been shattered by her parents' execution and her rape at the hands of the Falangists (part of the fascist coalition) at the outbreak of the war.

His strong sense of duty clashes with both the unwillingness of the guerrilla leader Pablo to commit to an operation that would endanger himself and his band and Jordan's own new-found lust for life, which arises from his love for María.

When another band of anti-fascist guerrillas, led by El Sordo, is surrounded and killed during a raid they conducted in support of Jordan's mission, Pablo steals the dynamite detonators and exploder, hoping to prevent the demolition and to avoid fascist reprisals.

Although he disposes of the detonators and exploder by throwing them down a gorge into the river, Pablo regrets abandoning his comrades and returns to assist in the operation.

Lacking the detonation equipment stolen by Pablo, Jordan devises an alternative method: exploding the dynamite by using hand grenades with wires attached so that their pins can be pulled from a distance.

He refuses Agustín's offer to shoot him and lies waiting in agony, hoping to kill an enemy officer and delay the pursuit of his comrades before he dies.

[15] The Oxford English Dictionary contains the phrase 'the earth moves' and describes it as 'used to refer to the experience of having an orgasm', 'first attested in For Whom The Bell Tolls by Ernest Hemingway'.

For example, Edmund Wilson, in a tepid review, noted the encumbrance of "a strange atmosphere of literary medievalism" in the relationship between Robert Jordan and Maria.