The history of Cuba during World War II begins in 1939. Because of Cuba's geographical position at the entrance of the Gulf of Mexico, Havana's role as the principal trading port in the West Indies, and the country's natural resources, Cuba was an important participant in the American Theater of World War II, and it was one of the greatest beneficiaries of the United States' Lend-Lease program. Cuba declared war on the Axis powers in December 1941, making it one of the first Latin American countries to enter the conflict. When the war ended in 1945, the Cuban military had developed a reputation of being the most efficient and co-operative Caribbean nation.

Federico Laredo Brú was the Cuban president when the war began. His only significant crisis related to the war before he left office in 1940 was the MS St. Louis affair. The MS St. Louis was a German ocean liner that was carrying over 900 Jewish refugees from Germany to Cuba. Upon her arrival in Havana, the Cuban government refused to allow the refugees to land because they did not have proper permits and visas. After ocean liner sailed north, the governments of both the United States and Canada also refused to accept the refugees and so the St. Louis sailed back across the Atlantic and dropped the passengers off in Europe. Some went to Britain, but most went to Belgium and France, which were soon overrun by German forces. Ultimately, because of the repeated refusal to take in the refugees, many of them were taken prisoner by the Germans and killed in concentration camps.

After the 1940 Cuban elections, Brú was succeeded by the "strongman and chief" of the Cuban Army, Fulgencio Batista. At first, the United States was concerned about Batista's intentions on whether he would align his country with the Axis or the Allies. Batista, shortly after becoming president, legalized a pro-fascist organization linked to Francisco Franco and his regime in Spain, but fear of any Nazi sympathies of Batista was dispelled when he sent the British a large quantity of sugar as a gift. Later, fear of any possible sympathy for Franco was also dispelled when he suggested to the United States that it launch a joint US-Latin American invasion of Spain to overthrow Franco and his regime, but the plan did not materialize.

Batista's support for the Allied cause was confirmed in February 1941, when he ordered all German and Italian consular officials to leave his country. Cuba entered the war on December 9, 1941, by declaring war on Japan, which had launched a devastating attack on the United States Navy base at Pearl Harbor, Hawaii, two days earlier. Cuba declared war on Germany and Italy on December 11, 1941 and, following the Americans, broke relations with Vichy France on November 10, 1942.

According to Rear Admiral Samuel Eliot Morison, Cuba's military was the "most cooperative and helpful of all the Caribbean states" during the war and its navy was "small but efficient" in its fight against German U-boats. Upon Cuba's declaration of war on the Axis powers, Batista signed an agreement with the United States that gave permission for the US to build airfields in Cuba for the protection of the Caribbean sealanes, and he also signed a mutual defense pact with Mexico to defend against enemy submarines in the Gulf of Mexico. Among the new American bases was the San Antonio Air Base near San Antonio de los Baños, and the San Julián Air Base, in Pinar del Rio, both of which were built in 1942 and turned over to the Cuban military after the end of the war. The United States also supplied Cuba with modern military aircraft, which were vital for coastal defense and anti-submarine operations, and refitted the Cuban Navy with modern weapons and other equipment.

During World War II, the Cuban Navy escorted hundreds of Allied ships through hostile waters, sailed nearly 400,000 miles on convoy and patrol duty, flew over 83,000 hours on convoy and patrol duty, and rescued over 200 U-boat victims from the sea, all without losing a single warship or aircraft to enemy action. However, even though the Cuban military was praised for its conduct, rumors persisted throughout the war that the Germans were operating small bases hidden in coves along Cuba's coast, which were used to resupply the U-boats. Nevertheless, the rumors were unjustified, and the lack of such bases in the Caribbean forced the Germans to develop supply submarines, the German Type XIV submarine, which was nicknamed "milk cows," for logistics.

Cuba lost six merchant ships during the war, and the Cuban Navy was credited with sinking one German submarine. The first four sunken merchant ships were the Manzanillo, a steamer of 1,025 tons, the 1,685-ton Santiago de Cuba, the 1,983-ton Mambi, and the 5,441-ton Libertad. Manzanillo was sunk with the Santiago de Cuba on August 12, 1942, by U-508. The two ships were sailing in Special Convoy 12 when they were attacked off the Florida Keys. Altogether, 33 sailors were killed in what became the deadliest attack on the Cuban merchant marine during the war, and 30 others survived.

The next engagement occurred on May 13, 1943, when U-176 sank Mambi. Mambi was with Convoy NC-18, sailing six miles off Manati, when she was hit by a single torpedo, sinking her quickly, killing 23 men, including five American United States Navy Armed Guards, who manned the ship's weapons; 11 others survived, including the ship's master and one of the armed guards. The 2,249-ton American ship SS Nickeliner was also sunk during the same attack after it had been struck by two torpedoes. The first torpedo explosion lifted the ship's bow out of the water and threw up a column of water and flames about 100 ft into the air. The second damaged the tanks of ammonia water that the ship was carrying. Miraculously, the crew, which included seven armed guards, escaped into lifeboats without a single loss of life. They were rescued by a Cuban submarine chaser as Nickeliner sank and landed at Nuevitas.