Pan Am was the survivor in a merger with National Airlines ("we fly coast to coast to coast") circa 1982. This actually made Pan Am a competitor with Delta and Eastern on a number of city pairs northeast to Florida and the gulf coast.

Pan Am and National had some nasty labor relations matters--I remember flying them DCA-PBI in 1983, on a 727, where we met Roger, who had been a captain on Pan Am, but, after the merger, was the flight engineer (that is, the 3rd man), with two ex-National guys in the front two seats. Roger ended up sitting in the back of coach talking to a friend (and to my wife and me), going back up to the front only when we got within 100 miles of PBI.

Later in the 1980s, when Frank Lorenzo had to give up the New York Air Shuttle when he was consolidating New York Air, People Express, Continental, the original Frontier and Eastern (I think Texas International was gone by then), Pan Am ended up with the NYA Shuttle, which is how DL ended up with it (after Eastern folded, US Scare ended up with what was once the Eastern Shuttle, then the Trump Shuttle)

When DL bought out Pan Am, they got the domestic system and Pan Am's TATL network except for London, which they had previously sold to United along with their TPAC network.