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We couldn't "do" any developed country in a war.
It's easy to win a first battle and declare the result, but modern warfare means never-ending wars. There would be no resolution.
We'd have to police a state with military force for as long as we wanted to hold it, and still we'd suffer some attrition.
All one can do is build enough of a deterrence that no-one comes knocking in the first place, but as an offensive capability it's all a bit daft.
Do you reckon we could do Spain in a war?