The United States during World War II was unprepared for one of Germany's most destructive war efforts: a U-boat assault on Allied ships in the Caribbean that sank about 400 tankers and merchant ships, with few losses to the German submarine fleet. The Germans had set up a network of spies and had the secret support of some dictators, including the Dominican Republic's Rafael Trujillo, supplying their U-boats with fuel.The Caribbean was of crucial strategic importance to the Allies. Roughly 95 percent of the oil sustaining the East Coast of the United States came from the region, along with bauxite, required to manufacture airplanes. The United States invested billions of dollars to build bases, landing strips, roads, and other military infrastructure on the Puerto Rico and secured a 99-year lease on all the British bases located in the Caribbean. The United States also struck an agreement with neutral Vichy France to keep the French Navy in the harbor of Martinique, preventing it from being turned over to the Germans, in exchange for a food supply for the island. Elsewhere, however, the German blockade was taking a dire human toll. All of the islands experienced a drastic food shortage. The US military buildup created jobs and income, but locals were paid a third as much as continental workers. The military also brought its segregationist policies to the islands, creating further tensions and resentment. The sacrifice of the Caribbean people was bitter, but their participation in the war effort was also decisive: The U-boat menace more or less disappeared from the region in late 1943, thanks to their work building up the US military operation.