Elon Musk's X/Twitter is suing a group of major companies, alleging that they unlawfully conspired to boycott the site.
It accuses the food giants Unilever and Mars, private healthcare company CVS Health, and renewable energy firm Orsted - along with a trade association called the World Federation of Advertisers (WFA) - of depriving it of "billions of dollars" in advertising revenue.
The lawsuit relates to the period in 2022 just after Mr Musk bought X, then known as Twitter, when advertising revenue dived.
Some companies had been wary of advertising on the platform amid concerns that its new owner was not serious enough about removing harmful online content.
X chief executive Linda Yaccarino said "people are hurt when the marketplace of ideas is constricted. No small group of people should monopolise what gets monetised".
Mr Musk tweeted: "We tried being nice for 2 years and got nothing but empty words. Now, it is war."
The WFA and the accused companies did not immediately respond to requests for comment.
Advertising revenue at X slumped by more than half in the year after Mr Musk bought the firm as advertisers avoided the platform.
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In its lawsuit, X alleges that the accused firms unfairly withheld spending by following safety standards set out by a WFA initiative called Global Alliance for Responsible Media (Garm).
Garm's stated aim is to "help the industry address the challenge of illegal or harmful content on digital media platforms and its monetisation via advertising".
By doing this, X claims the companies acted against their own economic self-interests in a conspiracy against the platform that breached US antitrust, or competition, law.
Bill Baer, who was assistant attorney general for the Department of Justice's antitrust division under Barack Obama, said the lawsuit was unlikely to succeed.
"As a general rule, a politically motivated boycott is not an antitrust violation. It is protected speech under our First Amendment," he said.
Professor Rebecca Haw Allensworth, of Vanderbilt University, said the boycott "was really trying to make a statement about X's policies and about their brands".
"That's protected by the First Amendment," she said.
Even if the case succeeds, the social media site cannot force companies to buy advertising space on the platform.
X is seeking unspecified damages and a court order against any continued efforts to conspire to withhold advertising spending.
It said in its lawsuit that it has applied brand-safety standards that are comparable to those of its competitors and "meet or exceed" those specified by Garm.
It also said X has become a "less effective competitor" in the sale of digital advertising.
The video-sharing company Rumble, which is favoured by right-wing influencers, made similar claims in a separate lawsuit against the World Federation of Advertisers on Tuesday.
