Obama in 2008 had already said he will not, under any circumstances, order a nuke strike on anyone. Our enemies are quite aware of that.
US policy has always been we will not respond in kind to any attack with weapons of mass destruction.
Nuke= we nuke you
Chemical= we nuke you
Bioligcal= we nuke you
Combination of NBC= we nuke you
That is still on the books somewhere