Delta ended interline agreement with American Airlines back in September 2015 (read more here) claiming that the number of passengers American moved to Delta was far greater than the other way around.
Seems that Delta has had a change of heart as the airline had approach American few months back to negotiate new agreement for irregular operations only. Airlines have now reached an agreement that is in effect as of today January 24, 2018.
Interline agreements allow airlines usually to sell tickets combining each others flights, check passengers and luggage through, and “interline” passengers at preferable rates during irregular operations.
Full service airlines usually have interline agreements with each others even when they otherwise fiercely compete (and sometimes bash) with each other.
Having these agreements in place better ensures that airlines can move passengers in case of delays and cancellations. It is in everyone’s best interest to get the passengers to their final destinations as soon as possible.
Delta sends some passengers to AA and AA sends some passengers to Delta (and both to other airlines and other airlines to them). In the long run this is just big zero sum game and just helps the entire system to function better and passengers happy.
Delta thought that their operations would be far superior to those of other airlines and they wouldn’t need the help of American Airlines at times. How wrong they were….