Many experts pointed to that decision as a primary source of its troubles.

“It’s very hard to operate a travel agency, and it is very hard to operate an airline,” said Mr. Kerby of the Travel Advisors association. “And the lessons you learn operating a successful agency do not always transfer to operating an airline. Both are independent, complicated businesses in their own ways.”

The aviation consultant John Strickland noted that carriers like Thomas Cook don’t have the flexibility of a stand-alone airline. “You aren’t offering the same kind of schedule options,” he said. The sunk costs of running an airline, combined with the operating costs — the crew, maintenance, and more — make any airline vulnerable to declines in demand. But Mr. Strickland said that Thomas Cook’s business model made it even more troublesome. “The nature of the market is that they are seeing big peaks in the summer and troughs in the winter,” he said. If you own your own planes and don’t have passengers during quieter months, “that becomes a big challenge.”

Thomas Cook’s chief competition was the German-based tour company TUI Group, which also has an airline. When the internet threatened to eat into the business of conventional tour operators, TUI executives started acquiring and operating cruise ships and hotels in an attempt to differentiate their agency from the competition. Now, in addition to a fleet of 150 airplanes flown by five company-owned airlines, TUI operates 17 ocean liners and 380 hotels mainly in Southern Europe and Southeast Asia.

“We’re getting 70 percent of our earnings from our cruise ships and hotels. Tour operations and airlines make up only 30 percent,” said Kuzy Alexander Esener, the head of media relations for TUI.

The loss of Thomas Cook is an opportunity for TUI, Mr. Esener said. Its TUI Fly airline has been contacted by the British authorities searching for jetliners to charter to bring Thomas Cook’s stranded passengers back home.

Problem #4: Brexit

In May, Thomas Cook’s chief executive, Mr. Fankhauser, warned that “the Brexit process has led many U.K. customers to delay their holiday plans for this summer.”

Mr. Kerby also blamed the uncertainty around Brexit, at least in part. He pointed to the particular difficulty of operating an airline in Europe, a market that is awash in low-cost carriers like Ryanair and easyJet, and where purchases need to be made in dollars. “A lot of your bills are in dollars; you have to buy oil in dollars,” he said. “So when there are shocks to the system, like the U.K. with Brexit and the pound losing so much value, all of a sudden that makes the loans you have very difficult to service.”