The first automotive torpedo was developed in 1866, and the torpedo boat was developed soon after. In 1898, while the Spanish–American War was being fought in the Caribbean and the Pacific, Assistant Secretary of the Navy Theodore Roosevelt wrote that the Spanish torpedo boat destroyers were the only threat to the American navy, and pushed for the acquisition of similar vessels. On 4 May 1898, the US Congress authorized the first sixteen torpedo boat destroyers and twelve seagoing torpedo boats for the United States Navy. In World War I, the U.S. Navy began mass-producing destroyers, laying 273 keels of the and destroyers. The peacetime years between 1919 and 1941 resulted in many of these flush deck destroyers being laid up. Additionally, treaties regulated destroyer construction. The 1500-ton destroyers built in the 1930s under the treaties had stability problems that limited expansion of their armament in World War II. During World War II, the United States began building larger 2100-ton destroyers with five-gun main batteries, but without stability problems. The first major warship produced by the U.S. Navy after World War II (and in the Cold War) were "frigates"—the ships were originally designated destroyer leaders but reclassified in 1975 as guided missile cruisers (except the became guided missile destroyers). These grew out of the last all-gun destroyers of the 1950s. In the middle 1970s the s entered service, optimized for anti-submarine warfare. A special class of guided missile destroyers was produced for the Shah of Iran, but due to the Iranian Revolution these ships could not be delivered and were added to the U.S. Navy. The , introduced in 1991, has been the U.S. Navy's only destroyer class in commission since 2005; construction continued through 2012 and was restarted in 2015. A further class, the , is entering service; the first ship was launched in 2013. The Zumwalt class will number three ships. Pre–World War I In 1864, US Navy Lt. William B. Cushing sank the ironclad using a "spar torpedo"—an explosive device mounted on a long pole and detonated underwater. Two years later in Austria, the British engineer Robert Whitehead developed a compressed air "automotive" torpedo; capable of over a distance of . The threat a small, fast, torpedo–delivering ship could pose to the battle line became clear to navies around the world; giving birth to the torpedo boat, including of the United States Navy. During the Spanish–American War, Assistant