The Federal Trade Commission on Monday accused Meta of creating a monopoly that squelched competition by buying start-ups that stood in its way, kicking off a landmark antitrust trial that could dismantle a social media empire that has transformed how the world connects online.

In a packed courtroom in the U.S. District Court of the District of Columbia, the F.T.C. opened its first antitrust trial under the Trump administration by arguing that Meta illegally cemented a monopoly in social networking by acquiring Instagram and WhatsApp when they were tiny start-ups. Those actions were part of a “buy-or-bury strategy,” the F.T.C. said.

Ultimately, the purchases coalesced Meta’s power, depriving consumers of other social networking options and edging out competition, the government said.

“For more than 100 years, American public policy has insisted firms must compete if they want to succeed," said Daniel Matheson, the F.T.C.’s lead litigator in the case, in his opening remarks. “The reason we are here is that Meta broke the deal.”

“They decided that competition was too hard and it would be easier to buy out their rivals than to compete with them,” he added.

The trial — Federal Trade Commission v. Meta Platforms — poses the most consequential threat to the business empire of Mark Zuckerberg, the company’s co-founder. If the government succeeds, the F.T.C. would most likely ask Meta to divest Instagram and WhatsApp, potentially shifting the way that Silicon Valley does business and altering a long pattern of big tech companies snapping up younger rivals.