If the government gets its way, it could force a breakup of the social-media giant. One analyst asks: “Without Instagram and WhatsApp, what really is Meta?”

Meta Platforms Inc. faces a historic antitrust trial beginning Monday that could force the tech giant to break off Instagram and WhatsApp, startups it bought more than a decade ago that have since grown into social media powerhouses.

WASHINGTON—Facebook parent Meta Platforms faces a high-stakes trial in Washington starting on Monday on claims it built an illegal social media monopoly by spending billions of dollars to acquire Instagram and WhatsApp, in a case where U.S. antitrust enforcers seek to unwind the deals.