When europeans arrived in 1500s and estimated 5 million people were living under'?

When Europeans arrived in the 1500s, an estimated 5 million people were living under Aztec. They formed a mighty empire by conquering adjacent cities. And they also controlled trade in the region and demanded tribute or payment from the cities they conquered.

  the domino theory states that  if one country in a region came under the influence of communism, then the surrounding countries would follow in a  domino  effect.