The ____ empire was formed in 1428 as an alliance of the city states Tenochtitlan, Tetzcoco, and Tlacopan (5)

I believe the answer is:

aztec

(Other definitions for aztec that I've seen before include "certain old people" , "one of Mexican Indian extraction" , "One into human sacrifice" , "Central American language" , "Old empire of Mexico" .)

I've seen this clue in The Sunday Times and The Times.

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