The Mk 2 defensive hand grenade (sometimes written Mk II) is a fragmentation hand grenade used by the U.S. armed forces during World War II and in later conflicts including the Vietnam War. The Mk II was introduced in 1918, replacing the failed Mk I of 1917, and was standardized in 1920. On 2 April 1945 the Mk II was redesignated the Mk 2.The Mk.2 was replaced by the M26-series (M26/M61/M57) and later M33 series (M33/M67). It was phased out gradually in service beginning with the Korean War. Due to the tremendous quantity manufactured during World War 2, it was in limited standard issue with the US Army and US Marine Corps throughout the 1950s and 1960s. The U.S. Navy was one of the last users when it was discontinued in the 1970s.