When US Airways and American Airlines announced plans earlier this year to combine, few expected the $11 billion deal to encounter antitrust turbulence. But last week, the ride got bumpy indeed when the U.S. Department of Justice, six states and the District of Columbia filed suit to block the merger.

For US Airways, it's familiar territory — the airline failed to secure the government's blessing in 2001 for its merger with United Airlines and abandoned the deal. The concerns raised then, albeit in a much different, pre-9/11 context, presaged objections to the deal now: How much consolidation is too much in the airline industry?