To understand why WWI started you must go beyond the killing of the heir to the Austro-Hungarian throne. During a time-period of about 40 years, a lot happened in Europe, growing bitterness between France, Germany and UK. Germany emerging as the new industrial leader and the colonization race. These factors among others created mistrust between the major nations and led to arms race wich led to alliance building.
Now this situation is the most "dangerous" in the international arena claimed by the realists (in the school of political science), a mulitpolar world with many "superpowers". That was the situation before WWI and it lead to a war which engaged nations beyond Europe. Now the realists claimed that a bipolar world with to superpowers would be most prudent, to superpowers would counter each other as seen between USA and USSR. After the fall of USSR we saw the unipolar world with USA as emerging superpower. Now, when a superpower you will try to keep that status, and that is whats leads me to the possible next conflict.
We see that other nations are growing stronger and stronger, most notably China, Russia and India. Now this again will lead to the multipolar world, every nation is dependent on non-renewable resources and they will be on constant search after new resources and pre-WWI they solved it with colonization but now this is not accepted. Today we can see USA in Iraq, China trying to increase its influnce in Africa most notably Sudan and India to some extent in Nigeria. Russia has more than enough with these resources for now. Second thing is Russia and USA will most defiently get in conflict in the northern areas, the north pole. Russia last year i think already planted their flag on the bottom of the sea to indicate that this "land" belongs to them. It is speculated that there is a lot of resources there. See example the Shtokman field in the Barents Sea. USA have also been aware of this, so let us see where this alone will lead us in the feature.
The other sign is of "alliance" building. We see a more aggresive expansion of NATO in the former Soviet nations of East-Europe to counter Russia and also steps in increased bilateral co-operation between USA and India to possible counter China. These are some signg i think which will show us how USA with NATO will be on the one side and Russia and China on the other side. I dont know if this will lead to Russia and China to co-operate. I think time will tell if EU gets tired of USA's lately unilateralism and develop a new agenda for themself.
As predicted by many scientist, freshwater as a resource alone will create many "small" warsconflict between nations in the future.
In the end, it is hard to predict who will fall and who will win, lets just hope that this war never appears if it does, it will be the most devestating.