The Meta chief executive, testifying in a landmark antitrust trial, denied he was trying to snuff out competitors.

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.

Boasberg will also be presiding over the trial and ruling for or against Meta, since there is no jury. His presence adds another layer of intrigue, since he also ruled against Trump’s use of the Alien Enemies Act to deport alleged Venezuelan gang members.