1898, as a result of the Spanish-American War, Spain ceded Puerto Rico to the United States by the Treaty of Paris. In 1900 the U.S. Congress accomplished a civil government on the island. U.S. citizenship was granted to Puerto Ricans in 1917, and the United States instituted measures designed to solve various economic and social problems of the overpopulated island. From 1940 to 1948 a hydroelectric-power development program was instituted to attract U.S. industry and to offer more employment for Puerto Ricans. Irrigation projects were also initiated. During World War II (1939-1945) the island became a key U.S. military base. Naval bases were constructed in San Juan harbor and on Culebra and Vieques.