[CathayRe]

I don't see how a War could happen between two Nations that has Nukes. Do they agree on not to use Nuke before the war?

There won't ever be a World War 3, if there is the next one might very well be the end of human civilisation.

[flukus]

Most nations have adopted a "no first use" doctrine, a promise not to use nukes until their enemy does. The only nuclear powers not to adopt this are the US and north Korea.