Russia and US are both nuclear superpowers. In the world, they are the only two who's got the most advanced of it. But while US has well-structured laws on the limits of its usage, Russia doesn't. Russia's nuclear power is their immediate defence. So any threat or attack addressed against them, it's lawful that they quickly retaliate with the nuclear equipment. Despite that US and Russia both have a nuclear equipment for defence, ones of Russia is more readily available to be used.