Question: what modern countries were once part of the aztec empire?
Answer: The Aztec Empire, or the Triple Alliance (Classical Nahuatl: Ēxcān Tlahtōlōyān, [ˈjéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥]), was an alliance of three Nahua altepetl city-states: Mexico-Tenochtitlan, Texcoco, and Tlacopan. ... 
The alliance waged wars of conquest and expanded rapidly after its formation.