The next potential deal in line - United and Continental - would be even bigger, and other pairings are likely too as carriers bulk up to compete in the new, more competitive global airline industry.
Executives at the big airlines believe that unless you have lots of planes flying at convenient times to many cities, valued corporate travelers won't remember you the next time they book a flight.
The economics of the airline industry also are driving carriers into each others' arms.
High fuel prices are causing many airlines to lose money, and the threat of recession makes the outlook even more grim. Airlines have raised fares, but not enough to offset fuel. Conventional wisdom holds that in mergers, airlines could reduce overlapping routes and raise prices.
Of the so-called legacy U.S. airlines - those that existed before deregulation and operate hub-and-spoke route networks
- Continental Airlines Inc. has been the most profitable since the industry downturn that began in 2001.
Continental's chief executive, Lawrence Kellner, says he would prefer to remain a stand-alone company but doesn't want to fall behind if others start merging.
"We do pay attention to relative size, and I think we would have some concern" if rivals merge, Kellner recently told analysts. He said he'll watch what competitors do, and "if we see something or hear something, we won't hesitate to act aggressively."
Northwest and Delta Air Lines Inc. have been talking about joining the two carriers, and people close to the talks say a deal could be announced as soon as next week.
In a memo to his employees Wednesday, Northwest Airlines Corp. CEO Doug Steenland said "consolidation is highly likely at some point," and doing nothing "could be our worst alternative."
"I do believe that consolidation is highly likely at some point - particularly with the high cost of fuel and the other challenges that the industry faces," he said.
If Delta Air Lines Inc., the nation's third-largest airline, combines with No. 5 Northwest Airlines Corp., it would create the largest U.S. airline, passing United and the current leader, American Airlines.
But United, the second-largest, and No. 4 Continental could trump Delta by joining forces.
The history of airline mergers is mixed at best. It's a challenge to combine different aircraft fleets, labor unions and cultures.
"Mergers are always a problem, and that's why Continental is not in favor of one," says Raymond Neidl, an analyst for Calyon Securities. "They have always been expensive and sloppy."
Neidl said network carriers believe they need bulk to compete with each other and with growing European and Asian airlines, especially when limits on U.S.-Europe service are eased at the end of March.
Continental and United operate complementary route networks, making a combination attractive. Continental has a strong presence in Latin America and, from its hub in Newark, N.J., flying to Europe. United has strong routes across the Pacific, including in the growing Chinese market.
A Continental-United partnership wouldn't have as much global reach as Delta and Northwest
- an important consideration for business travelers - "but it would certainly surpass either carrier by themselves and also American. It's not a bad second choice," said Robert Mann, an airline industry consultant.
Other potential partners for Continental and United "aren't in the same ballpark," he said. Many analysts believe antitrust concerns could prevent American from buying another big carrier.
Potential roadblocks to a deal could include the new company's name, whether it would be based at Continental's home in Houston or United's base in Chicago, and who would run it. Leadership issues reportedly have been a sticking point in the Delta-Northwest talks.
