Even in the absence of an applicable statute or court rule, failure to assert an available counterclaim precludes bringing a subsequent action thereon if granting relief would nullify the judgment in the initial action. This so-called common-law compulsory counterclaim rule emerges from the intuitive principle of claim preclusion that a valid and final judgment generally precludes the defendant from later asserting mere defenses to the claim. The implicit extension of this idea is that once a plaintiff obtains a judgment, the defendant generally cannot bring a new action to undo the judgment by reopening the plaintiff’s claim and pushing those defenses. The evident rationale is that claim preclusion simply must apply when the effect of the defendant’s collaterally asserted defense would be to nullify the earlier judgment for the plaintiff. This implicit barrier to collateral attack may seem to occupy some arcane corner of the specialty of res judicata. But the common-law compulsory counterclaim rule in fact is critical to any judicial system. That is, although it is intuitive, it is also important. The rule applies whether or not the prior judgment was by default; the rule indeed is especially important because it works to guarantee that even default judgments mean something and cannot normally be undone by later litigation. The rule applies whether or not a compulsory counterclaim statute or rule of court exists; that fact explains why its very name declares it to be a common-law doctrine. This article takes a wide view of the rule’s purposes and development to determine the rule’s proper scope.