Hubbing and being the dominant carrier at the US cities that are biggest for international traffic: EWR (for NYC), SFO, ORD and to an extent even IAD.

Delta has primarily JFK and LAX, and to a smaller extent BOS and SEA but both LAX and BOS are only recent developments (in the case of LAX, even though they have had large operations there for a while, they have only had a renewed focus on it in the last few years). And in NYC, even though they have the more well-known New York airport for international travel, they are handicapped by the split between JFK and LGA whereas UA is able to consolidate everything under one roof in EWR.

AA on the other hand has arguably the fewest hubs of the Big Three that are large international travel markets: Miami and Chicago (though playing second fiddle to UA in the latter).