Zanzibar Land is the military state in which the events of Metal Gear 2: Solid Snake take place. It's location is somewhere in a Central Asian region, between Afghanistan and Tajikistan. This roughly corresponds with the region where Metal Gear Solid 3: Snake Eater also takes place. Zanzibar is also referenced in Metal Gear Solid.
As a former province of the USSR, it gained its independence during "The Mercenary War," becoming a haven for guns for hire. After this war, the state became a heavily militarized nation, and through raids of nuclear waste facilities acquired the means to build nuclear weapons.
During this time, Big Boss seized control of this nation and made it a home for his private army, which included mercenaries such as Gray Fox and Black Ninja.
In 1999, Solid Snake managed to kill Big Boss during a covert-ops mission, Operation Intrude N313 (the events of Metal Gear 2: Solid Snake). After the death of Big Boss, the army collapsed, and the military nation fell apart.