There's evidence that dinosaurs existed in a higher oxygen environment, allowing their large bodies to exist.

There are claims that a large space object impacted our planet long ago.

Many religions record a great flood event in prehistory.

So. I suggest that a great hydrogen comet impacted earth. This event created mass quantities of water and soot, resulting in mass excitinction caused by mass flooding and oxygen drop.

Stink bugs have propogated their way across the USA in the last 5-7 years, and there's a variety of other bugs, fish, crustaceans, and even rodents that are prevalent that simply didn't exist here a decade or two or three ago.

Obviously some are accidental stow aways in cargo ships etc, but all of them? Why not force your enemy to deal with a common plague that your country already has?