What is the war in V for Vendetta?

In the background setting in the graphic novel that they sourced the story from, England is the only known survivor of a full nuclear war that destroyed every other major nation (for the very simple reason that they became isolationist and failed to be a target when the time came). The attempts by England to survive the apocalpyse led to the rise of Norsefire, as Evey relates early in the comic.

In the film, possibly because the likelihood of England or the world surviving even a limited nuclear winter is near nil, this was changed to a virus plague that swept through. According to V, that virus was secretly released by Norsefire themselves, in order to engineer their own rise to power with a ready-made cure.

The rest of the world is not elaborated on in the film. Norsefire has a decent propoganda machine, as pointed out by cde in comment, so really anything's possible. It can be speculated that the virus was more loose that Norsefire intended, or that other groups in other countries attempted a similar fear-tactic to gain control, to mixed results. Either way, the end result would be the same as in the comic - collapse of the world order, and England on it's own.