On Tuesday, X Corporation, formerly known as Twitter, sued “Operation Bluebird,” the new startup that is seeking to reclaim the allegedly abandoned Twitter trademark and relaunch a new social media network under that name.

In its 43-page lawsuit, which was filed in federal court in Delaware, X Corporation alleges trademark infringement, adding that despite Bluebird’s “purported plan, it cannot bring Twitter ‘back’—Twitter never left and continues to be exclusively owned by X Corp.”

One of Bluebird’s leaders, Michael Peroff, told Ars in an email that Operation Bluebird was “fully expecting” a lawsuit from X Corporation and that “we planned for it.”

Stephen Coates, also of Operation Bluebird (and who formerly served as Twitter’s general counsel), reiterated to Ars that X “legally abandoned the TWITTER mark.”

“Our cancellation petition is based on well-established trademark law and we believe we will be successful,” he wrote in an emailed statement. “They said goodbye. We say hello.”

Neither X Corporation, nor its attorneys, nor CEO Elon Musk, immediately responded to Ars’ request for comment.

“Twitter is one of the world’s most recognized brands, and it belongs to X Corp,” X Corp’s lawsuit continues. “Simply put, a rebrand is not an abandonment of trademark rights.”

Josh Gerben, a Washington, DC-based trademark attorney who has been closely following this case, told Ars that proving that X Corporation has “abandoned” the trademark is a “huge uphill battle—they’re going to have to climb the equivalent of Mt. Everest.”