As a legal matter, a conspiracy involves two or more people agreeing to commit an illegal or unlawful act and, then, one or more of the conspirators taking at least one overt step or action towards the completion of such act. The steps or actions taken towards completion of the intended illegal or unlawful act need not themselves be illegal or unlawful.Posted by Ernie Nemeth (here)
But what if there was no conspiracy, just the theory that there was one?
Conspiracy to commit a crime, coupled with at least one overt act in furtherance thereof, is itself a crime, even if the planned underlying crime is not carried out for whatever reason.
Consider: if I agree with at least one other person to rob a bank and under the agreement I am to lawfully obtain the getaway car and the other is to lawfully obtain a handgun, and we both do that and get in the car and drive to the bank with the intent to rob it, but before we rob the bank, someone else, who we do not know and never spoke to, robs the bank have I and my co-conspirator committed a crime in fact? In theory? Not at all?
Who would know of the conspiracy if we keep our mouths shut? But, even if no one knows, did we commit a crime and get away with it?
Should conspiracy be a crime standing alone?
By the way, there is such a thing as civil conspiracy where the wrongful act is not illegal or criminal, but rather wrongful as, for example, it harms someone’s civil interests.