Antitrust is thought by some to be the bulwark of free enterprise. Without the continued vigilance of the Justice Department and the Federal Trade Commission, so the argument goes, large corporations would ruthlessly destroy their smaller rivals and soon raise prices and profits at consumers’ expense.

But antitrust has a dark side. The time for modest reform of antitrust policy has passed. Root-and-branch overhaul of what Alan Greenspan referred to a generation ago as a ‘‘jumble of economic irrationality and ignorance’’—and what modern scholarship has shown repeatedly to be a playground of special pleaders—is called for.