I don't know if a true world was could happen without some kind of major event that brought China into the fold. The US could probably role over anyone else on the planet in a week if they used their full capabilities.
World War 3 is imo already happening and has been for awhile but it's not happening between 2 factions of armies, it's happening everywhere on the Internet and from the shadows. The fight for our minds and the information that we receive is the new frontier of conflict. Look at Russia and Putin's approval rating. Most are probably just too scared to oppose anything, but a shocking number of Russians are like minded in his "Russia for Russians" ideology and are completely engulfed in the propaganda.
Social media and other distractions have flooded our information streams and no one digest the right media needed for democracies to function anymore.
I couldn't believe the voter apathy in Ontario this year. I had so many friends tell me they didn't bother voting because they weren't informed on the candidates. They couldn't be bothered to spend an hour reading some platforms and 10 minutes to check off a box. It's sickening but it isn't happening by accident. You're not supposed to care and if you do care you've been conditioned to treat politics like sports, you have your home team and will support them without question.
This war is over capital and between a ruling class and the citizens it pretends to represent or provide services for. As religion dies off in the developed world as the prime tool of control for the ruling class, media and the Internet are taking its place. The Internet could be our salvation or it could lead to the apocalypse. That has yet to be decided, but we're currently losing this war as people like Obama and Harper do their best to kill our online freedoms and track us 24/7.