Civilians associated with a national entity which conducted a WMD attack against US civilians. Could be a nation-state, or even someone like hamas. Basically if you don't want to get nuked, better not let your government conduct WMD attacks against major nuclear powers (whatever you have to do to prevent that).

Worth noting that a WMD attack *might* be something like a mass cyber attack, ie shut down the economy, power, water utilities, and vital supply delivery. We do not have a no first use policy (not that those are worth the paper they're printed on anyway).