Ummm....no biggie.... there are already bombs hugely more nasty than their atomic WW2 little brothers. Why the hype???
Those are typically attached to ICBM delivery systems. Fire off one of those (or a flock of them) and response is guaranteed. But stealth aircraft and gravity bombs of sufficient power may render response impossible or limited. They're gone before they know they were under attack.
Please note that this is NOT a first strike recommendation, but acknowledging the option.