First, you have to define what World War is. If you are talking about the tradition image of massive barrages of nuclear missiles screaming through the sky is not going to happen for many reasons. Let us get to the two scenarios that could affect the US as it current stands.
First, a single nuclear missile/bomb attack. If/When the US gets hit a nuke then US will go to war against whoever did it but they will NOT respond with nukes of their own. The international community and the American people would not stand for it and what would you target. It wouldn't be a city full of civilians and why hit some random military base with a nuke?
Second, a full nuke strike can only come from one target currently, and they would be destroyed either by our nukes or their economy would collapse. And if some other nation was able to make enough nukes, to take out the US they would destroy the world's environment in the process and it would not matter what the US did in response, even if the US was able to.

The world is always full of war and the only thing that distinguishes the two World Wars from any others is the sheer number of nations, mostly European, that were involved and thankfully those European nations are tired of war, for now at least. There is always some war of aggression, civil war or revolution going on and it is never going to stop. You either send in troops from more powerful nation to keep the peace, ignore it and let thousands or millions of people die or go in and kill everybody with a gun and hope that the next people who pick up the gun has better sense to solve their differences with a pen.


Saber rattling has always been an important element of diplomacy. We threaten Iran with bombs, they threaten with terrorism. We say we will not allow it, they say we cannot stop them. While this is going on in public, behind the scenes diplomats are hashing out an agreement.
The US military cannot handle a war with Iran right now and Iran knows this and is using it as diplomatic leverage. They want more power and to be on par with Europe and the US, which I cannot blame them.