If there is another World War, chances are it would come from the Balkans area of Europe, considering that two previous wars have come from there... As for who would be involved it would definitely involve the EU. France might be on the opposite side from Britain and hopefully the rest of the EU going by its recent form in opposing measures like Iraq and its underhand dealings there... Kosovo, Bosnia, and Yugoslavia, Gorgia are all places where there is a lot of tension that could spill over at any time...
No that will not be the cause of a World War. It could very easily break out again into fighitng. I spent 10 months deployed to Kosovo keeping the Serbs from crossing the border and the Albanians and Serbs from killing themselves in Kosovo, they still want at each other throats. There are still hundreds of years of hatred for each other brewing in there hearts.
If they were to start fighting again, all that would happen is NATO an d the EU goes right in and bombs the **** out of them and reoccupies the area. The people there will not fight NATO or the EU once it is over because they are actually happy to have them there. They can go to work and school without fear of mass genocide.
Serbia as well does not have the military capability to deal with NATO or the EU. They would just be beated back just like they were before.