From Sony's game page
Stories tell of a legendary solider who once waged war against the world. His name was Naked Snake, a.k.a. Big Boss, and the truth behind this legend will finally be told. The era is the 1970’s. Big Boss had stationed his soldiers in Colombia, South America, where he was approached by visitors from Costa Rica, a "nation without a military." In the shadow of the Cuban Missile Crisis, as Latin America became a key to maintaining the power balance between East and West, Costa Rica had miraculously managed to maintain its peace and neutrality. However, in various regions of the country, a mysterious armed force seemed to be engaging in suspicious activities. Now, to preserve peace in this nation without a military, the his soldiers move into action. In time, they would be known as the founders of "Outer Heaven." Forge a new path and live the legend with METAL GEAR SOLID PEACE WALKER, the first official Hideo Kojima game on the PSP system.