The Fractional Orbital Bombardment System (sometimes known as FOBS) is the stepchild of physics, strategy and politics. It was designed and built by the Soviet Union during the Cold War, and decommissioned in 1983. Its name reflects the legal nitpicking around its nature.
In 1967, the world was quite familiar with ICBMs. They were the premier means of threatening atomic attack against other nations. However, their use (theoretical or actual) was subject to a number of challenges. On the one hand, they had limited range, if sufficient. For another, they generally were (and are) easy to see coming - their use in direct great-circle ballistic arcs means that your opponent has a very narrowly defined arc of sky to monitor in order to be relatively certain that your attack will be detected as it comes over the horizon. In the early 1960s, the USSR was at a disadvantage - it had less capable SLBMs than the West, and needed another way to avoid the BMEWS and DEW line systems. Finally, the West had (via forward basing in Europe and Asia) been able to bring a larger variety of suborbital weapons into regions where they could threaten Soviet targets.
The obvious answer to this, given the success that both the East and West had enjoyed in spaceflight, was to simply use an orbital weapon. Warheads placed in earth orbit could in theory be directed to circumnavigate the globe in most any direction, depending on how much energy you were willing to expend, and then deorbited in such a manner as to cause them to land on their targets. However, in 1967, the U.S. and USSR signed the Outer Space Treaty which forbade the placing of nuclear weapons in outer space - orbit or beyond.
It said nothing about either weapons on the ground, however, or about weapons which did not complete full orbits (like ICBMs) and thus weren't considered to be 'in outer space.' The Soviets, therefore, pursued a ground-based system which could on launch place a nuclear warhead in a variety of orbits such that these orbits would, prior to a full circumnavigation, bring the warhead over their targets in the United States. At that point, systems on the spacecraft would deorbit the warhead down onto its target. Thus, since these weapons were only to be launched in time of use, and since even in testing they did not place nuclear weapons stably in space, they did not violate the terms of the Outer Space Treaty.
After going through a few design variants, the Strategic Rocket Forces brought online a new missile (named the R-36Orb, for Orbital) in 1969. That missile, a variant of the SS-18 (R-36) ICBM, was capable of lofting its 8K69 warhead into a full low earth orbit, and then deorbiting over the US. It contained radar altimeters to determine its orbital parameters, and a solid fuel rocket which both drove a turbine to power its systems and whose exhaust was used in thrusters for maneuvering and deorbit burn. Since the weapon was designed to terminate the orbit prior to a full circumnavigation, it remained within the treaty boundaries. It was larger and more expensive than the ICBMs of the day, of course, as it required much more energy to reach its intended flight path. The system was referred to in English translation as FOBS, or Fractional Orbital (because it didn't go all the way around) Bombardment.
The system was deactivated in order to comply with the SALT treaty. By 1983, none of the R-36Orb missiles remained in active service, although several of the boosters were retained and later repurposed for civilian space launch purposes under the name Tsyklon (Cyclone).