The United States Department of Justice (DOJ) on Thursday announced the filing of a civil antitrust lawsuit that seeks to stop Visa Inc.’s $5.3 billion acquisition of Plaid Inc.

Visa is a monopolist in online debit services, charging consumers and merchants billions of dollars in fees each year to process online payments. Plaid, a successful fintech firm, is developing a payments platform that would challenge Visa’s monopoly.

The complaint alleges that Visa’s CEO viewed the acquisition as an “insurance policy” to protect against a “threat to our important US debit business.” This acquisition is the second-largest in Visa’s history, with an extraordinary price tag of $5.3 billion. Visa’s CEO justified the deal to Visa’s Board of Directors as a “strategic, not financial” move, and noted that in part because “our US debit business is critical and we must always do what it takes to protect this business.”

Unless acquired, Visa feared that Plaid “on their own or owned by a competitor was going to create some threat” with a “potential downside risk of $300-500M in our US debit business” by 2024. If Plaid remained free to develop its competing payment platform, then “Visa may be forced to accept lower margins or not have a competitive offering.”

The complaint alleges that Visa has dominated online debit for years and has protected its monopoly with exclusionary tactics that have prevented rivals, including Mastercard, from expanding or entering. The lawsuit alleges that Visa’s proposed acquisition of Plaid is a violation of both Section 2 of the Sherman Act and Section 7 of the Clayton Act.