NOT LONG AGO, it looked like the U.S. airline industry might be reduced to three major carriers: American, United and Delta. But those forecasts underestimated the global forces at work in the airline business.

That seems to be the case at Continental Airlines, which said Monday it will accept an offer by Air Canada and a Texas firm to invest $450 million in the airline. This is the third time in recent years that a foreign airline has shown interest in a U.S. carrier. KLM Royal Dutch Airlines is already part owner of Northwest Airlines and British Airways is bidding for a piece of USAir.It has never made much sense to restrict foreign investment in a business as international as aviation. As airlines forge cross-border ties, they will only become stronger, which is good news for them and their passengers.

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