Short Cough

Every so often (usually right after that old Strangelove movie gets revived) someone puts for a proposal for the Earth Government to build a Last Resort device, something that would poison or destroy the Earth and as much of the surrounding space as possible, to act as a deterrent for invasion.

And while enough civilisations believe we'd use such a device, the issue is always that if we build it, someone will try to set it off instead.

So ideally, people would think we had one, but we actually didn't.

But that seemed hard, so Earth just made one anyway.