That's how nukes work. When it comes to nuclear weapons, the world is divided into haves and have-nots. Anyone lacking effective nuclear response can be steamrolled by those who do with total impunity.
The USA has been attacked before but it has never been invaded and forced to fight a war on its own soil against foreign enemies. It's possible that they unconsciously believe war is something they bring to others, never something others bring to them. It's impossible to predict how traumatizing it would be for them if that belief is proven wrong. They could absolutely reach for nuclear weapons if that threshold is reached.
The USA has been attacked before but it has never been invaded and forced to fight a war on its own soil against foreign enemies. It's possible that they unconsciously believe war is something they bring to others, never something others bring to them. It's impossible to predict how traumatizing it would be for them if that belief is proven wrong. They could absolutely reach for nuclear weapons if that threshold is reached.