And then US and Russia both have MIRV systems which carry 15-20 warheads on one rocket, so if you don't intercept it in the first stage of flight it's game over anyway. And Russia recently finished developing the System-6 nuclear torpedo which can destroy possibly more land than large megaton warheads anyway, since it creates a 500m tsunami wave which travels hundreds of miles in-land destroying everything in its path. And if it's salted with cobalt then that land becomes unusable for decades.