All the reasons (global economic crisis stimulating international competition, nationalisms) and mechanisms (countries grouped into camps like EU and America vs Iran-China-Russia - which would pull one after another into the conflict) for the world war are in place most of the time. Just a spark is enough to trigger it - it's a complex system with self-organised criticality. Or is the self-organised critically fad over? Anyway, triggering the conflict is IMHO not a matter of the spark being big enough but about giving it many enough tries. Maybe it's 1 per 1000 now and was 1 per 100 before the previous WWs - probably it decreased, but if we keep trying the fire will start.
Paul, I'm not at all hysterical about it... *plucks the strings of the fiddle*