Every time Southwest begins service in a new airport, it raises the threat that Southwest will enter routes connecting that airport with other airports in its network. We illustrate this in Figure I. Southwest enters Washington Dulles in the fourth quarter of 2006 and immediately begins flights to Chicago Midway. Southwest is already flying out of Cleveland to other points in its network besides Dulles. Now, though, its entry into Dulles makes Southwest much more likely to start flying the IAD-CLE route in the near future.6 Actually, even the announcement of the initial entry into Dulles ought to indicate to incumbents that the probability of future entry has risen