The USS San Diego (ACR-6) was a protected cruiser of the United States Navy, launched on March 9, 1904. It was the first ship named after the city of San Diego, California. The vessel was designed for various roles, including reconnaissance and fleet support, and was equipped with advanced armament for its time. Commissioned in 1906, the USS San Diego served in several naval operations, including World War I. It was notable for being the only U.S. Navy ship sunk by an enemy submarine during the war, when it was torpedoed by the German submarine U-156 on July 19, 1918.