If you want to really know where the next wars are going to be, buy the latest edition of
A Quick & Dirty Guide to War: Briefings on Present & Potential Wars by James Dunnigan. He's proven pretty accurate in the past, and gives a whole lot of background as to why the conflicts are predicted to happen in each chapter. He also has
a whole website of wonky military articles of lesser utility.
A major reason why countries go to war is overpopulation. And all the models are saying pretty much it's a given that the next World War will be between the two most fucking over populated countries in the world, India and China. It is highly, highly likely that the elites in both countries, when threatened by the underclass trying to rise up in either one of them, start a nuclear war between the two that wipes out mostly of their massive lower classes (while leaving the elites untouched). It'll be similar to the way Khomeini used the Iran-Iraq war to rid Iran of retarded and otherwise unwanted children
by sending them in body waves of certain death to clear mine fields.
The next world war will involve over 50% of the world's population. But it will be largely between only two countries, and probably over so quickly every else will wonder what the hell happened. Japan may get caught in the crossfire, as possibly other neighbors. But Europe and the Americas won't be able too do much except watch from a distance, shoot down any stray missiles launched their way, and it'll be over before anyone outside the region even realizes it started.
There is the slight possibility it will turn out to be a conventional war, but not in the western sense. Both sides can easily mobilize armies so large that there isn't enough refinery capacity in the entire world to fuel trucks to drive that many people. It would start with modern mechanized warfare, but within a few weeks both sides will be reduced to Napoleonic-level foot marching and huge supply problems even getting ammo and food moved to the battle front. Some believe it could even degrade back to swords and pikes as the only weapons still able to function.
In the more likely nuclear scenario, the real trouble will be dealing with the radioactive fallout that will be in the atmosphere for an unknown amount of time afterwards --no one has set that many nukes off at once in a limited area before. Some models also include the possibility that both sides use "cleaner" neutron bombs, and other WMD instead of old fashioned atomic bombs. It's still not pretty. Most likely both sides use their remaining air-forces (and fuel) not to engage either other, but to spray chemical aerosol poisons across large swaths of the each other, crop dusting each others (non-elite) civilians into further oblivion.
So while it's unlikely that anyone on this forum will be serving in the next world war (or there will even be time to draft, train and fly them over before it's finished), we'll all be breathing the fallout and poisons left behind for years, possibly decades, afterwards.