"WWIV will be fought with sticks and stones."

"I know not with what weapons WWIII will be fought, but WWIV will be fought with sticks and stones."

- Albert Einstein

Have you ever thought how amazing technology is? We have so many things now. Today when I entered my Chemistry room (where I originally drafted this), a bright red fluffy feather was sitting on my desk. We are now able to produce a modified replica of a rare biological occurrence. We can get to the other side of the planet in a day (excluding stop-offs). 6000 years ago, that would have been only enough to walk 72km (assuming the human walking pace is 3km/h, ish). I mean, you are able to read this. Hell, most people reading this wouldn't even know me if it weren't for technology (♥ mah interwebz buds).

And I guess I don't even need to go into that much detail about the quote, but I will anyway. We all know about how technology has bought us guns, bombs, nuclear weapons. Sure, there is the Non-Nuclear Proliferation Treaty, but treaties are mere paper. You can bet that if any one country/group fired their nukes, the rest would follow suit. (See End Of The World animation for a humourous take on this.)
Thus, nuclear winter, therefore, humanz go bewm (quite literally). Not only humans, but pretty much all life on Earth. Even if we survived, we'd be set back to the stone age.
Technology is still developing though. And it's pretty sure that WWIII will be big...
"War in our blood, some things never change." - Avenged Sevenfold

My conclusion can best be expressed by a quote from Arnold J Rimmer (Season 1, Ep 2): "Old Albo and I happen to agree on this one..."
WWIV will be fought with sticks and stones.