Law.

Common law conspiracy is an agreement by two or more to do an unlawful act, or to do a lawful act by unlawful means. The actus reus (guilty act) is the agreement itself. The mens rea (guilty mind) is the intention to carry out the unlawful act. Another way of putting it is that there must be a "meeting of the minds" to commit the unlawful act.

It does not matter that the conspiracy was not carried out, liability for common law conspiracy arises simply from the the agreement.

It goes almost without saying, that to avoid First Amendment protection of a hypothetical Espionage case (and I will leave those arguments to US constitutional lawyers), conspiracy appears to have a much greater prospect of success for the DOJ, provided evidence is forthcoming that can satisfy all the required elements of conspiracy.