(To see all the posts in this series, click here: Naval Armor.)
In part three of our survey of the evolution of naval armor, let’s look at the pinnacle of the heavily armored ship, the World War II battleship. The last battleship commissioned by the US Navy was the USS Missouri (BB-63). The Missouri – nicknamed “Mighty Mo” – was numbered the third of four Iowa-class battleships, but was the last one completed. She was launched January 29, 1944, at the height of World War II.

The USS Monitor, which we discussed in Part 2, was a radically new design, an untested experiment which during the pressure of war went from laying the keel to launch in 118 days. The Missouri on the other hand was the culmination of all of our years of experience and evolutionary development of big-gun, heavy-armor warships. Her main armament was nine 16-inch “Mark 7” guns, arranged in three turrets of three guns each. Each individual Mark 7 is 66 feet long and weighs 267,900 pounds with the breech. A Mark 7 can hurl a high-explosive shell weighing 2,700 pounds (a 1969 Volkswagen Beetle weighs 1,764 pounds) some 24 miles, and land it in an area the size of a tennis court. Just one salvo from one turret exceeded the normal 6,000 pound bomb capacity of a Boeing B-17 Flying Fortress.
As for armor, she carried 7.5 inch steel deck armor, a 12.1 inch armor belt to protect against torpedoes, and her gunners enjoyed 19.7 inch armor on the turrets. Her turrets had nearly the thickness of the USS Constitution’s sides, but with steel, not oak. This was not even the most armor seen during the war. The largest battleship ever, the Imperial Japanese Navy’s Yamato, had 26 inch armor on the face of her main turrets.
Remember in Part 2 the dents in the turret of the Monitor, caused by Confederate shells? The Missouri also suffered a dent in her armor, caused not by shells but by a Japanese Zero in a kamikaze attack on April 11, 1945. The dent remains to this day.
The Missouri participated in many of the famous battles of the Pacific in World War II: Iwo Jima, Okinawa, the Japanese home islands. She became famous as the site of the Japanese surrender ceremony on September 2, 1945 while anchored in Tokyo Bay, ending World War II. The Missouri fought again in Korea, supporting the Incheon Landings, screening aircraft carriers and bombarding North Korean positions. With the end of the Korean war, she was mothballed (put into the reserve fleet) in 1955 after only 11 years of service. There she remained until 1984 when she was reactivated and modernized as part of the “600 ship Navy” program. She last fought in Operation Desert Storm – the 1990 Gulf War in Iraq. During this fight she had far greater firepower than the Mark 7 guns, for during her modernization she was fitted to launch “Tomahawk” cruise missiles. On March 31, 1992 she left active service again – as the last United States battleship to be decommissioned. Today she is a museum ship anchored at Pearl Harbor, Hawaii, watching over the USS Arizona Memorial. It is fitting that our last battleship resides at Pearl Harbor, because the Japanese carrier attack of December 7, 1941 proved conclusively that the battleship had been superseded. The aircraft carrier, not the battleship, was to be the future means of projecting naval power.

Sources:
[1] USS Missouri (BB-63), 1944-1998, Selected Views, Naval Historical Center
[2] USS Missouri BB-63, Wikipedia
[3] Battleship Missouri Memorial, USSMissouri.com