On Tuesday, X (formerly twitter) filed a lawsuit in federal court in Texas against the Global Alliance for Responsible Media (GARM), a coalition of major advertisers, claiming that GARM had violated antitrust laws by coordinating with brands to deter them from spending money on the social media platform.

The suit claims that the coalition, identified as GARM, “conspired” with top brands, including CVS, Mars, Unilever and the Danish energy company Orsted, to “collectively withhold billions of dollars in advertising revenue” that were owed to X, then named Twitter, in the wake of Elon Musk’s acquisition of the social media company in 2022.

“The illegal behavior of these organizations and their executives cost X billions of dollars,” wrote X’s CEO (Linda Yaccarino), in an open letter to advertisers. “People are hurt when the marketplace of ideas is undermined and some viewpoints are not funded over others as part of an illegal boycott.”

With the litigation, X effectively declared war on advertisers, which afford the bulk of the social media behemoth’s revenue. Since Mr. Musk bought the company and promised to usher in a new age of unfettered free speech, many advertisers have limited their expenditure on X, concerned by reports of growing hate speech and misinformation there. By taking legal action against GARM, Musk continued to break with the heads of other social media companies, who have established close relationships with advertisers and been quite responsive to their concerns about offensive online content.

“We tried being nice for 2 years and got nothing but empty words,” Mr. Musk wrote in an X post on Tuesday. “Now, it is war.” He added in a different post that he encouraged any enterprise that faced a boycott to file a lawsuit.

“To the extent that Elon hadn’t already burned all bridges and ties with the entire advertising community, I don’t see how this will get any advertisers to come back to X,” said Ruben Schreurs (chief strategy officer at a marketing and media consulting firm, Ebiquity). “It’s a last-ditch effort to force brands who don’t want to be in the cross hairs of this kind of legal action to return to the platform.”

GARM, which represents key brands that are responsible for more than 90 percent of global advertising spending, advised advertisers to shun X after Mr. Musk bought it. In the wake of the buyout, 18 GARM members stopped advertising on X altogether, according to the complaint. Dozens of others reduced their spending by 70 percent or more, the filing said.

X’s revenue has been hit severely. In the second quarter of this year, X earned $114 million in revenue in the US, a 25 percent drop from the first quarter and a 53 percent drop from the same period the previous year, as per the internal documents obtained by The New York Times. The company wishes to reach $190 million in U.S. revenue during the third quarter, boosted by advertising linked with the Olympics, football and political campaigns, the documents said — but that mark would still set X’s quarterly earnings at 25 percent lower than they were last year.

Because of reduced advertiser expenditure, X was forced to lower the price of its ads, the lawsuit said. But even though its ads were cheap compared with other social media platforms, advertisers did not come back.