The airline industry's next top-of-the-card battle will likely break out next quarter in Denver, when Frontier Airlines will seek to dramatically expand and United Continental Holdings Inc. (UAL - Get Report) will try to make that difficult.

It's the type of clash that has long captivated industry observers and analysts, as an established global hub carrier fights an upstart, which in this case is one of the two principal ultra-low-cost carriers.

It features two well-known industry veterans, United President Scott Kirby and Frontier CEO Barry Biffle. Both worked at US Airways, one before the bankruptcies and one afterward. They have battled before, although not directly.

In 2015, American Airlines Group Inc., where Kirby was president, cut fares aggressively in response to rapid growth by Spirit Airlines Inc. (SAVE - Get Report) at American's hubs in Dallas and Chicago. Biffle had left Spirit in 2013, but was a key architect of the carrier's Dallas and Chicago expansions.

Biffle "was at Spirit for at least the start of the DFW expansion if not most of it -- he was definitely the architect of the strategy," said a former airline executive who asked not to be named.

Now, both Biffle and Kirby "have moved to new teams and different hubs, but otherwise they've laced it up and are about to duke it out," the former executive said. "They even used similar phrases, each talking about 'natural share' or some variant. This is good for consumers, not sure about shareholders."

The stage was set last week for a broader conflict in Denver.

Frontier said on July 19 that it would expand its network, adding 21 destinations at Denver International Airport, where it is the third-largest carrier after United and Southwest Airlines Co. (LUV - Get Report)  . The move "signals a renewed focus on Frontier's primary hub after several years of reductions at DIA," The Denver Post reported.

Two days later, during United's second-quarter earnings call, Kirby responded to a question about Frontier by saying, "Having watched the ULCC growth over the last decade this is the best news that I've heard in the last 10 years.

"They're pivoting from what has been the most successful models, point-to-point ULCC strategy around the world, to going back to trying to copy what the network carriers do," he said.

Hub operation is far tougher than point-to-point, Kirby said. "You've got to staff up, because you have peaks and valleys {and} you've got to connect bags, which is one of the most operationally difficult thing we do." Also, he said, flights must operate on time because many passengers connect.

"Competing on our turf and trying to get a network carrier in Denver, that is a battle I guarantee United will win," he added.

Frontier said Kirby has it wrong. "Frontier currently offers connections over Denver, and always has, so this isn't a change in the business," the carrier said in a prepared statement.

Aviation consultant Bob Mann said, "First it was Kirby dissing Spirit over the DFW buildup, and now it's Kirby dissing Frontier over its Denver bulk-up and return to network operations to supplement its point-to-point operations."

"With Frontier's ULCC costs and hybrid business model now bulking up at Denver, my sense is United and Southwest have the problem, not Frontier," Mann said.

"Of course, Frontier has to execute its expansion well and {provide} reliability," he said. "It seems to me that is the risk Frontier faces, but it's not one United can influence."

On the financial side, something is going on at Frontier. Bloomberg News reported on July 19 that the carrier pushed back its initial public offering, originally planned for the second quarter. Frontier is owned by Indigo Partners, which is headed by veteran airline investor Bill Franke.

Frontier was created in 1994, after Continental closed its Denver hub. In 2006, Southwest entered the Denver market; in 2008, Frontier filed for bankruptcy protection. Republic acquired Frontier in 2009 and sold it to Indigo Partners in 2013.

Last week, Frontier began selling tickets from Denver to Albuquerque, N.M; Oklahoma City, Okla.; Ontario, Calif.; Palm Springs, Calif.; Reno, Nev.; and San Jose.

New spring 2018 destinations from Denver include Boise, Idaho; Buffalo, N.Y.; Calgary, Alberta; Charleston, S.C.; El Paso, Texas; Fargo, N.D.; Fresno, Calif.; Grand Rapids, Mich.; Jackson Hole, Wyo.; Jacksonville, Fla.; Little Rock, Ark.; Louisville, Ky.; Pensacola, Fla.; Spokane, Wash.; and Tulsa, Okla.

Speaking at United's 2016 investor day, Kirby said Denver is United's most profitable hub, a result of low airport costs and a high level of connecting revenue.

Year to date, United has a 42% market share at Denver International Airport, while Southwest has 29%, Frontier has 11%, Delta has 6% and American has 5%, according to airport statistics.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.