The plea of res judicata is a defense available in conspiracy cases[i]. Although the defense of res judicata appears to be similar to the defense of former jeopardy, the former differs from the later in its legal implications.
An order of acquittal passed in a former case will operate as res judicata on a subsequent case only if the matter in the former case was tried and adjudicated properly[ii]. A verdict can be said to be a properly adjudicated one only if the issues involved in the case were fully considered and determined.
An order of acquittal made in a prosecution of a crime will not operate as res judicata to bar the subsequent prosecution for conspiracy to commit that crime. However an order of acquittal resulting from a prosecution for conspiracy of a crime will operate as a bar on the prosecution for the commission of the crime on the ground that there is re-litigation of the same question of fact that was already determined in the previous prosecution. Whereas if an order of acquittal results from a conspiracy prosecution where the prosecution attorney was unable to convince the jury on the conspiracy, this proceeding will not operate as res judicata to a subsequent proceeding to prosecute the commission of the crime[iii].
However, an order of acquittal for conspiracy will operate as a bar to a subsequent proceeding against the defendant for aiding and abetting the commission of a crime, if evidence of the agreement between the abettor/defendant and the abetted is essential to prove the crime[iv].
[i] People v. Ricker, 45 Ill. 2d 562 (Ill. 1970)
[ii] United States v. Smith, 4 U.S.C.M.A. 369 (C.M.A. 1954)
[iii] United States v. Calabro, 449 F.2d 885 (2d Cir. N.Y. 1971)
[iv] Zambrana v. United States, 790 F. Supp. 838 (N.D. Ind. 1992)