The United States entered World War II on December 7 1941 and for the first time in his life, Hemingway took an active part in a war. Aboard the Pilar, now a Q-Ship, he was ready to fight and sink Nazi submarines threatening the coasts of Cuba and the USA. It is worth noting that, according to Anthony Burgess, he never before shot nor would have shot another human being, and that he was a non-combatant in World War I, in the Sino-Japanese War he was reporting on after having written "For Whom the Bell Tolls" and in the Spanish Civil War, where even the money he collected to support the Loyalists was used on non-belligerent purposes. Perhaps his failure in preventing the Fascists from taking Spain (he was very possessive about this country) had led him to take more drastic actions.
As the FBI took over the Caribbean counter-espionage, he was disbanded and went to Europe as war correspondent for Collier's. At Ville-dieu-les-PoÍles he threw three grenades into a cellar where SS men were hiding, a clear violation of the Geneva Convention and his first murder. Seemingly encouraged by that, he declared he would be an unofficial intelligence unit. Later, he acted as an unofficial liaison officer at Rambouillet, and afterwards, he even formed his own partisan group which took part in the liberation of Paris. He tried to step further onto the path of the warrior the personages of his fiction, in this case particularly Pablo, had taken before him.
By firing his machine pistol at the portrait of Mary Welsh's husband after having placed it atop of the toilet bowl in his room in the Ritz, he proved he wouldn't any longer flinch from killing a man who stood face to face with him. He became a killer like Pablo in the end.