Southwest doesn't fly to Atlanta right now, but AirTran operates about 200 flights a day there.
Executive Vice President Bob Jordan said Thursday that Southwest will augment AirTran's Atlanta service with flights to 20 or more cities where AirTran doesn't fly.
"It's easy to get to kind of 250-plus" daily flights while still being a distant second to Delta Air Lines Inc. in Atlanta, Jordan said.
Las Vegas is Southwest's top city, with 224 daily departures as of Aug. 15.
Southwest made inroads against US Airways in Philadelphia and is taking on United in Denver. Now it will go head-to-head against Delta, the world's second-largest airline.
"You would assume that there is no way they would just sit back and do nothing," Jordan said. "This is going to be a fight."
Southwest announced last month it would buy AirTran Holdings Inc. for $1.4 billion.