Britney Spears -- Fires Back at Iggy Azalea ... At Least I Have Shows!

Britney Spears/Iggy Azalea's feud has reached Defcon 1 ... and Britney is taking shots at Iggy's failed tour ... while promoting her own.

After Iggy bashed Spears over their failed song, "Pretty Girls," Spears cleverly shot back, "Can't wait to get back to Vegas. So thankful I have shows for the rest of the year to look forward to... #YouWantAPieceOfMe."

You'll recall ... Iggy said, "I don't have to suck the womans a**hole 24/7 to be her friend."

Spears is no doubt making reference to the fact Iggy pulled the plug on her 2015 Great Escape arena tour in May.

Who doesn't love a good catfight?