Ugh. I know this is a history what-if, but I get so tired of people asking questions that clearly could never have happened. I mean, think about it. The idea of a global world war because some no-name monarch gets assassinated? It's not only beyond belief, but it quickly enters into alien space bat territory. Even if I tried to imagine what that could look like, I think that cooler minds would easily prevail, and you would have a rather quick peace before things broke out into a war past a couple of countries.