There have been quite a few world wars in rp, ive only been doing it since December 2010 so i dont know many. However there was a terrible war between Alpha's Prussia and Barrick's Grand Power, which ended up in the surrender of Grand Power, but Barrick remained in charge for a couple of months until he resigned and his deputy roomatt gave grand power to prussia. There was a uprising in grand power but it was easily crushed by prussia.
Also, there was a world war between Prussia and OGN (Old Guards of Napoleon) but it wasnt as wide as the prussia-GP war and OGN soon lost due to unwilling generals and a numerically superior force there was almost another world war called the prusso-bavaric affair in which bavaria denied prussias requests to advance forward in roleplay time and hence prusia attempted to annex bavarias lands and a coalition rose on either sides to aid each party it was soon resolved diplomatically however
There have also been a wars against the most wanted man in RolePlay:Fruition