In 1519, Tenochtitlan may have been the largest city in the world. Tenochtitlan itself probably had a population of several hundred thousand, with another million or so in the cities in and around the lake. Great pyramids and palaces dominated the city's central precinct, and thousands of people visited the great market at Tlatelolco every day.
The hub of a great military empire, Tenochtitlan was built in the middle of a lake. Access was by canoe or along four causeways, each several miles long, with bridges that could be quickly dismantled to prevent entrance to or escape from the city. The temerity of the conquistadors to think that they could exert control over these people in this place was unfathomable, and yet that is what they proceeded to do.
When they met on the southern causeway, Cortés tried to give Moctezuma a friendly abrazo, but he was stopped. Nobody touched the emperor. Commoners and minor nobility were not even allowed to look at the great man, but Cortés wanted to give him a hug!
Moctezuma put up his guests in the lavish palace of his predecessor. He treated them with great courtesy, and showed Cortés around town. He took him to the top of the pyramid at Tlatelolco. Look at this mighty city, he may have said, certain that his guest would be awed and humbled.
The conquistadors well knew how precarious their position was and after a couple of weeks Cortés resolved to try a bold stratagem. Using a minor incident perpetrated by one of Moctezuma's vassals on Cortés' company at Villa Rica, he placed the emperor under house arrest! For the remaining months of his life, although he continued to conduct state business, receive emissaries and be waited on by his lavish staff, he did so under Cortés' watchful eye.
Why did the great Moctezuma put up with this indignity? I don't think it was out of fear. Although the Spaniards had amply demonstrated their ferocity and technical superiority, they were few and the Aztecs were many. Moctezuma had but to snap his fingers and a hundred thousand warriors, trained from birth to expect and even desire death in battle, would have risen against the intruders. In fact, Moctezuma had to use considerable persuasion with his councilors and generals to prevent them from doing just that. And he himself was a distinguished warrior familiar with the joys of battle.
But he seems to have had a premonition, whether from superstition or astute analysis, that great, inevitable changes were in store for his world. (A deep sense of inevitability permeated Aztec culture.) He seems to have hoped, from the strangers' first appearance until his own violent death, that once Cortés got enough gold he would take it and go.
Gold there was. The pile of loot that Cortés had stacked in front of him surpassed the treasury of any European power of the day, and although it was soon to be dwarfed by the treasures Pizzaro would steal from the Incas and much of it would be lost in the fighting to come, Cortés' share would make him one of the world's richest men.
NOTE: Quotes from Díaz and Cortés are from the following sources:
The Conquest of New Spain, Bernal Díaz del Castillo. Translated by J. M. Cohen. Penguin Books, 1963.
Letters from Mexico, Hernán Cortés. Translated by Anthony Pagden. Yale University Press, 1986.