It was thanks to the last major merger attempt that resulted in BAEC not earning miles and TPs on AA on trans-atlantic routes and vice versa.
Mind you, as anyone actually paid close attention to the routes that AA fly in the US. Its OK for flights from JFK to SFO or LAX. However, internally, it does not really have a really good domestic timetable. Really bad for domestic flights along the West Coast and most connections to Cental/Latin America are from Miami not LAX.
Most importantly, AA tend to operate its domestic flights except for trans-continental flight use LGA leaving international flights from JFK. Thats the problem with any BA / AA link.