Warsaw Pact and NATO nations almost entirely. The war ended very abruptly with an appeal to mutually assured destruction: "If we go any further, all we will achieve is wiping each other out completely." Nations in the second and third worlds are almost untouched, as the nuclear weapons were targeted at population-dense areas for terror and morale purposes, and at vital infrastructure for the purpose of crippling the other side economically. So although, for example, the USA was already in the Middle East in the nineties, the worst the soldiers would receive there is the fallout carried on the winds from Western Europe. There aren't enough factories or power plants there to make the Middle East a target for crippling the USA's war efforts.

So there's a very shaky peace across the developed world ever since the Cold War thawed out. Basically we found a new Holocaust to grieve over, and everyone agreed unanimously to call it quits once the first footage of ground zero leaked. Stalkers are viewed by the outside world as madmen for [i]choosing[/i] to enter these horrible hells which are now well-documented on TV.