Any large chain which comes into a fresh retail environment can underprice their competition because they have an economy of scale which sometimes permits them to sell items for less than smaller stores can stock them.
A price war, in and of itself, is not an antitrust violation, and neither are "loss leaders." Selling below cost with the intent to drive a competitor out of business, however, is called predatory pricing and is a violation of Section 2 of the Sherman Act). I think there is pretty widespread agreement amongst economists that predatory pricing is impossible in most real world situations, but it nevertheless remains illegal.
Any large chain which comes into a fresh retail environment can underprice their competition because they have an economy of scale which sometimes permits them to sell items for less than smaller stores can stock them.
Agreed, but that isn't selling at a loss, though.