The reason ww3 won't happen is because the world power, the USA, the EU, The Soviets err Russian, the Chinese have no real reason to fight a war with each other. The superpowers have carved out power and figure its better this way then trying to fight another superpower for more.
It's more like a bunch of little wars that never really end more then 1 big one that involves many countries. The countries that would like to attack Israel just not powerful enough Gaff it would be suicide and over in months.
I just don't see a ww3. Maybe another Iraq but thats about it. But Obama won't put boots on the ground so.......