conspire: to join in a secret agreement to do an unlawful or wrongful act or an act which becomes unlawful as a result of the secret agreement

Yes I suppose Germans (and others) who thought Hitler was working on a 'secret weapon' were holding to a conspiracy theory. That conspiracy theory happens to have been correct as I understand it (not all conspiracy theories are incorrect, right?); the Germans were working on atomic weapons (though did not succeed).

Not sure why you (I infer) think this example a contradiction. Perhaps because you don't think the Nazis working on an atomic weapon to destroy the Allied Powers was a wrongful act. It most certainly was. But then again lotsa things the Nazis did were wrong. The Nazis' existence in power was wrong (even if it arose constitutionally), and much follows from that easily.

caveat - I may not be adhering to The World's def. of conspiracy theory in any of this

--Blixa