Twitter on Monday said that Elon Musk's effort to abandon his $44 billion takeover of the company is "invalid and wrongful," according to a letter filed with the U.S. Securities & Exchange Commission.

Why it matters: This is an opening legal salvo in what soon will become a full-fledged brawl in Delaware Chancery Court.

What Twitter said: The social media company, via its attorneys, claims that it's "breached none of its obligations" under the merger agreement, and that it has not suffered a "material adverse effect."

• Musk on Friday argued that the company had breached its obligations by not providing certain data about "bot" accounts, and by several HR moves that he deemed to have not been in the course of normal business.
• Twitter also claims that the only contractual breaches have been made by Musk, and that they have been done "knowingly, intentionally, willfully, and materially."
• It concludes by saying that the merger agreement "is not terminated."

What to watch: Expect Twitter to file its formal complaint against Musk later this week.

Go deeper: The biggest losers from Musk's Twitter chaos