IMO, there is almost no chance of any super powers going head to head on each others' land. It will be like Syria or Vietnam, where some other country suffers. Having the battle occur on ones own land suppresses the population's support of the war. It's one reason (the main one?) we dropped nukes on Japan -- to turn public opinion against their leadership's wishes to continue fighting. Fighting remotely makes the war a purely political proposition for the opposing sides with only members of the military and their families seen as directly affected.
Having said that, use of chemical and biological weapons is against international law. If the US used such a weapon, we'd have to do so very covertly -- in a way that can't be tied to our military. No US president wants to answer for warcrime charges.
No, we are in an age of unconventional warfare. Cyberattacks, economic attacks, and disruption of the political processes will have more success than direct armed conflict. It's like two competing corporations using every trick they can to cause the other to fail without resorting to an overt and direct assault.
When was the last time the US was invaded? Last week -- from immigrants marching through Mexico. That's one tactic I see as being not only unanswered, but assisted, by the Liberal Leftist Socialist elites now in power. But, it's subtle. Overwhelming our economy, welfare, judicial, educational and infrastructure resources with waves of immigrants who can easily create an untenable situation to deal with. Great Britain is seeing that firsthand with the immigrant population there. What if many of the immigrants are being promised compensation to gather information or engage in other acts that undermine our security? Security of electric grids, transportation, elections, food & water supplies, etc.
It's been long predicted that our Republic would not fall to a foreign power, but through people within working against the country and taking advantage of the freedoms, liberties and laws we created to keep us safe from government oppression.