Countries with nuclear weapons should not go to war with each other.
Nuclear powers are burdened with a far greater need for restraint out of necessity than non-nuclear powers, even in the face of provocation.
And yet, despite knowing that their cause was hopeless, and that the war would be unwinnable, the government of Imperial Japan knowingly committed suicide, by attacking the United States in 1941.
This is one of the reasons I find the principles underpinning MAD to be fairy tales. In matters of war, belligerents do not behave rationally.