Perhaps the most famous Little John in history was Robin Hood's sidekick. You know, the guy responsible for driving the Sheriff of Nottingham and Prince John crazy. But the United States Army had a "Little John" of their own that was designed to give airborne units, like the 82nd and the 101st, one heck of a punch.
During the 1950s, the Army had used the Pentomic structure for a number of its infantry divisions — a structure designed to fight in nuclear war. A problem remained within light units, however: They needed nuclear firepower to hold off hordes of Soviet tanks, but the nukes of the time were bulky things.
By 1959, though, the answer had emerged: the M51, which the Army called "Little John." It wasn't based on the Robin Hood legend, though. The name was chosen since the Army had a self-propelled nuclear rocket launcher called the Honest John, and the Little John, as you might guess, was a smaller rocket system.
The Little John was eventually re-designated MGR-3. It had a range of about 11 miles and was unguided. Well, when a missile is packing a W45 thermonuclear warhead with a yield of 10 kilotons (a kiloton is equal to the explosive force of 1,000 tons of TNT) and has a speed of Mach 1.5, you don't really need guidance to take out a target.
The MGR-3 stuck around for about a decade, but in 1969, they were quickly retired. The rockets had successfully held the line, but when an even smaller, lighter tactical nuclear weapon was developed — one that could fit inside the shell fired by a typical 155mm howitzer — Little John became redundant. That replacement was the W48 warhead.
See how the Army described the Little John as it entered service in the video below: