Puts aren't a good way to hedge against this because in the event the US empire does collapse, your US dollars won't be of much use. In fact, I'd bet that the stock market will actually shoot up to insane levels because the government response will be more money printing to fund military endeavors.

Probably better to invest in land in places with a (relatively) stable climate, good fresh water supply, and far enough north and high enough in elevation to be protected from the most obvious effects of climate change.