Most of the tension is left over from the Cold War, when US and Russia was trying to out produce and out demonstrate. Pretty much the equivalent of you and your neighbor throwing sticks of dynamite in your back yards, demonstrating to the other that you're more than able to blow up each others' houses. Second, it's a huge amount of destructive power able to be deployed virtually immediately, with no proven countermeasures. Third, they are weapons that largely do not target military installations exclusively. There's really no avoiding hitting civilians. Nukes can never win wars. At best, they are a reason not to fight, but that doesn't really seem to be the case.