Aztec Timeline By Lila.popelier 1428 Aztecs formed alliance Texcoco and Tlacopan in Mexico 1440 Moctezuma I became the supreme emperor 1500 Aztec ruled 500 small city-states 1501 Moctezuma II became the supreme emperor 1519 Cortes sailed to Veracruz in the Gulf of Mexico after hearing rumors about it 1521 Aztec is overwhelmed by disease. 40,000 Aztecs died.