Other controversy relating to the Toltecs include how best to understand reasons behind the perceived similarities in architecture and iconography between the archaeological site of Tula and the Maya site of Chichén Itzá – no consensus has emerged yet about the degree or direction of influence between the two sites. The Nahua peoples began to enter central Mexico in the 6th century AD. By the 12th century, they had established their center at Azcapotzalco, the city of the Tepanecs. The Mexica people arrived in the Valley of Mexico in 1248 AD. They had migrated from the deserts north of the Rio Grande over a period traditionally said to have been 100 years. They may have thought of themselves as the heirs to the prestigious civilizations that had preceded them. What the Aztec initially lacked in political power, they made up for with ambition and military skill. In 1325, they established the biggest city in the world at that time, Tenochtitlan. Aztec religion was based on the belief in the continual need for regular offering of human blood to keep their deities beneficent; to meet this need, the Aztec sacrificed thousands of people. This belief is thought to have been common throughout the Nahuatl people. To acquire captives in times of peace, the Aztec resorted to a form of ritual warfare called flower war. The Tlaxcalteca, among other Nahuatl nations, were forced into such wars. In 1428, the Aztec led a war against their rulers from the city of Azcapotzalco, which had subjugated most of the Valley of Mexico's peoples. The revolt was successful, and the Aztecs became the rulers of central Mexico as the leaders of the Triple Alliance. The alliance was composed of the city-states of Tenochtitlan, Texcoco, and Tlacopan. At their peak, 350,000 Aztec presided over a wealthy tribute-empire comprising 10 million people, almost half of Mexico's estimated population of 24 million. Their empire stretched from ocean to ocean, and extended into Central America. The westward expansion of the empire was halted by a devastating military defeat at the hands of the Purepecha (who possessed weapons made of copper). The empire relied upon a system of taxation (of goods and services), which were collected through an elaborate bureaucracy of tax collectors, courts, civil servants, and local officials who were installed as loyalists to the Triple Alliance. By 1519, the Aztec capital, Mexico-Tenochtitlan, the site of modern-day Mexico City, was one of the largest cities in the world, with an estimated population between 200,000 and 300,000.