It is easy to get lost in the thicket of the Volcker rule, even though the true measure of its merit doesn't lie in the hundreds of pages of details. Rather, its significance will rest on the rule being a milestone along the path toward ending the threat of too-big-to-fail banks, not a capstone to postfinancial-crisis overhauls.

Critics of the rule approved by regulators Tuesday point out that trading mishaps weren't the root cause of the financial crisis. That, to a degree, is true. If so, they argue, Volcker is a misguided attempt to shackle banks.