"Fraud vitiates every thing, and a judgment equally with a contract -- that is, a judgment obtained directly by fraud, and not merely a judgment founded on a fraudulent instrument; for in general the court will not go again into the merits of an action for the purpose of detecting and annulling the fraud. . . . Likewise, there are few exceptions to the rule that equity will not go behind the judgment to interpose in the cause itself, but only when there was some hindrance besides the negligence of the defendant in presenting the defense in the legal action. There is an old case in South Carolina to the effect that fraud in obtaining a bill of sale would justify equitable interference as to the judgment obtained thereon. But I judge it stands almost or quite alone, and has no weight as a precedent."
United States v. Throckmorton https://supreme.justia.com/cases/federal/us/98/61/
"Fraud vitiates every thing, and a judgment equally with a contract -- that is, a judgment obtained directly by fraud, and not merely a judgment founded on a fraudulent instrument; for in general the court will not go again into the merits of an action for the purpose of detecting and annulling the fraud. . . . Likewise, there are few exceptions to the rule that equity will not go behind the judgment to interpose in the cause itself, but only when there was some hindrance besides the negligence of the defendant in presenting the defense in the legal action. There is an old case in South Carolina to the effect that fraud in obtaining a bill of sale would justify equitable interference as to the judgment obtained thereon. But I judge it stands almost or quite alone, and has no weight as a precedent."