United Airlines President Scott Kirby said Wednesday that Frontier Airlines adding dozens of new routes at the key Denver hub for both is a competition that United can win.

“I can promise you they are now competing on our turf trying to be a network carrier in Denver,” Kirby told investment analysts on a quarterly earnings call. “That is a battle I guarantee United will win.”

Kirby said he’s been predicting for a decade that ultra-low-cost carriers such as Frontier couldn’t compete on price if legacy carriers such as United adopted strategies such as Basic Economy for cheaper tickets.

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Frontier announced Tuesday it would expand to 21 new destinations, including 16 that it had previously offered service to. Among the new cities joining Frontier's route map: Buffalo; Pensacola, Fla.; and Ontario, Calif. Passengers at the carrier’s home airport in Denver passengers will have about 20 new destinations and overall the airline will have 1,000 new connecting options by summer 2018.

“Customers will benefit not only from the broad new selection of nonstop routes, but our growing network will provide more than 1,000 new connecting route options,” Frontier CEO Barry Biffle says in a statement.

Frontier has long operated a hub at Denver, where it has historically gone up United's competing hub there. But Frontier's Denver operation encountered new pressure when Southwest entered the market in 2006. Denver quickly became one of Southwest's fastest-growing cities, putting Frontier into a competitive squeeze between a large network carrier (United) and a strong low-cost airline (Southwest).

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Frontier eventually filed for bankruptcy protection amid the Great Recession and then recast itself as an "ultra low-cost carrier" (ULCC) this decade. In doing so, Frontier focused more on growing outside of Denver with "point-to-point" routes that catered to passengers seeking non-stop options. Now, with its Tuesday announcement, Frontier appears to be inching back toward a connecting model for much of its network.

But operating like a network carrier such as American, Delta and United is much more complicated than point-to-point service that Frontier had offered, according to Kirby. He predicted it might prove to be be a difficult track for a ULCC like Frontier.

“They’re pivoting from what had been the most successful model of point to point ULCC strategy around the world to going back to try to copy what the network carriers do, which is run a connecting business model,” Kirby said. “This is the first – I guess – public validation that one of the ULCCs is throwing in the towel on the point to point business model and switching to a network model.”

In a network that's heavy on connecting flights, plane utilization slows down because airlines must ensure that passengers and crews make their connections. Baggage must also connect, which Kirby said is one of the most difficult things that airlines do.

If a flight from Orlando to Denver runs two hours late, Frontier now simply flies later and has few broader repercussions. But in a network, half the people on the plane – and their luggage – could be looking for connections, Kirby said.

With one flight a day, Frontier will have a choice of delaying its entire network by two hours, paying for passengers to say in a hotel or paying a rival like United to fly them to their connection, Kirby said.

“It is exponentially more complex to run a connecting model,” Kirby said.