people obviously don't understand, this is KISHI'S story, tobi tells us that a 4th world war would have started, that means a 4th war WOULD have happened, this is kishi's way of letting us know why itachi did what he did.
and itachi was 4 years old, and was seeing dead bodies everywhere, itachi was still trying to reason on both sides. the elders didn't want to have to slaughter the clan, they knew they would have suffered great losses and also put themselves on the weak side of a WORLD WAR. itachi knows they would have slaughtered the children and old as well. so by his own hands he killed them to save millions later, the kids would have turned evil and just reattempted the same thing since their whole clan was gone. and tobi helped as well, heck it's speculation, but tobi probably just warped all the uchiha he killed, including kids and elderly, itachi might not even have lifted a finger to the kids.