Oh stop pulling an OP. Unnaturally high levels of CO2 can be dangerous after long term exposure - hours in an industrial environment that already has a bunch of other crap in the air. Carbon Monoxide from your home furnace at high levels knocks you out and kills you in 30 minutes. Still not the same.
America is more than ready, considering how much we spend on the military. This article is a few years old, and definitely sensationalist, but is still an interesting read-
https://www.wearethemighty.com/mighty-trending/how-long-the-us-military-would-last-against-the-rest-of-the-world/
And on top of that, the US Army just started deploying robotic troop carriers, and is finishing up remote combat vehicles for the front lines so that first contact isn’t with soldiers. Yes, we’re going to have killer robots on the front line. And soon.