In irony that surely won’t register in Washington, the latest crackdown on Google comes just as the internet giant is showing—on its own—that it is far from invincible.

The Justice Department is preparing a new antitrust investigation against Google parent Alphabet Inc. This comes six years after a similar probe from the Federal Trade Commission, which resulted in no significant damage to the company that powers more than 90% of the world’s internet search activity. The Wall Street Journal reported that the two agencies held...