“The U.S. market is a mature market — we’ve seen the shakeout, we’ve had consolidation,” said Henry Harteveldt, a travel analyst with Forrester Research. “To continue to grow, airlines have to expand abroad, but it’s impractical and, in some cases, impossible for carriers to do much expansion on their own. That’s why we’ve seen the alliances become so popular.”

International treaties dictate where carriers are allowed to operate in another country, and laws in the United States prohibit foreign ownership of a majority stake in an airline, so global alliances are also a way of expanding abroad without a buyout or a merger.

But now that the dominant carriers in each alliance have been granted antitrust immunity by the United States government and the European Union, they are free to behave as if there had been a merger — coordinating schedules and fares and sharing revenue on trans-Atlantic routes. The alliances also help the airlines save money as members shift operations to a single terminal and move toward more joint lounges and services.

“The one-roof strategy we have in airports helps save costs,” said Marie-Joseph Malé, SkyTeam’s managing director. “In London, we are all in Terminal 4. If we were separated, we would have had 60 check-in counters, which we were able to reduce to 48.”

Mr. Malé pointed to the liberalization of international aviation treaties, the globalization of the economy and advances in technology as the three main factors contributing to the growing strength of airline alliances. SkyTeam currently has 13 members, including Delta, Air France and KLM, and plans to have 19 by the end of 2012 — focusing on increasing its presence in China.