In 1968, the U.S. introduced a light tank, the Sheridan, which fired a guided missile from its 152mm main gun. This Shillelagh missile (with a max range to 2,000 meters), was not a success, and Sheridans sent to Vietnam just carried conventional rounds.
In the 1970s, the Russians began to introduce guided missiles fired from 125mm tank guns. The Russian missiles (there were six different models introduced, the latest in 2001, with another still in development), all had a max range of 5,000 meters and, like Shillelagh, used a shaped charge warhead. None have been used in combat.
The MRM KE/CE comes in two versions. The KE (Kinetic Energy) uses a metal penetrator, and gets a speed boost from a rocket motor to give it sufficient momentum to penetrate thick armor. The CE (Chemical Energy) round uses a shaped charge. The MRM uses GPS, radar and a laser to find its target.
Current U.S. 120mm tank guns can get hits at up to 4,000 meters, using chemical (explosive) rounds. The vast majority of enemy combat vehicles spotted are within that range. But it is expected that greater use of UAVs will enable tanks to identify enemy vehicles at longer ranges, and the MRM KE/CE shells would enable these targets to be hit.
While Russia has been firing guided missiles from tank guns for 30 years, the United States has only recently gotten back into the business. A new "guided shell" (the MRM KE/CE) has had several successful test firings this year. In the latest test, a U.S. M-1 tank fired an MRM CE round, which hit a moving T-72 tank at a range of 8,600 meters.