How would a world-wide WMD ban even happen? The last remaining country with WMDs could just ramp up their WMD production to Cold War levels and then hold the rest of the world hostage. North Korea is proof that sanctions can't stop nuclear problems.

The only plausible route I imagine is if the non-WMD states put massive resources into countermeasures (e.g. missile interceptors or thwarting nuclear terrorism).