JetBlue Airways on Thursday announced it would purchase Spirit Airlines,  a combination that would create America's fifth-largest airline.

JetBlue had been pursuing a hostile bid for Spirit even while Spirit sought shareholder approval for a lower-priced deal with Frontier.

The proposed JetBlue Spirit deal is smaller than many airline mergers of recent decades, which turned the 10 largest US airlines into four mega-carriers that control 80% of US air traffic.

But the Biden administration has taken a much more aggressive stance on questions of antitrust law and vowed to promote greater competition within the airline industry.

Biden's Justice Department sued to block an alliance between American and JetBlue that allows each airline to book passengers onto the other's flights.