That's a really difficult question to answer, but it probably would be because one group of countries is no longer willing to tolerate the shortcomings of another and their feud would spill outside borders and trigger a world war.
Of course there is also the philosophy that we are already fighting world war III and the cause is terrorism triggered by the fall of the World Trade Centers. If one thinks about it, there are very few countries currently at peace with the world. Almost everyone is fighting with someone somewhere currently. In these cases and in the case of most wars, philosophical differences lead countries to find excuses to fight with each other.
Just my 2 cents