After Noche Triste or "the Night of Sorrows," Cortés' forces barely escaped. Cortés, at the head of the expedition out of Tenochtitlan, survived. Pedro de Alvarado, somewhere in the middle, was rumored to be the last one out. Others died, trapped in the city.

Even outside the city, the Spaniards faced fierce struggles and lost many more before they reached a place of safety near Tlaxcalan territory. Even there they were not necessarily welcomed and complained about having to pay for their food and water with their gold and other jewels they had escaped with.

Things dramatically changed for the Spaniards when the Tlaxcala chiefs, Xicotencatl and Masse Escase arrived to reassure them that they were loyal allies. They helped the Spaniards to heal their wounds and them accompanied them to Tlaxcala, where they planned to retake Tenochtitlan.

Though the Spaniards had superior weapons and the horses, they recognized that they were of little use during their last days in the Mexica capital. The Mexicas had fortified their rooftops, which allowed them to throw spears and barbed darts at the Spaniards without much loss for themselves.

The entire construction of Tenochtitlan gave them further advantages. The city was constructed on an island in the middle of Lake Texcoco. It was connected to the mainland by three large causeways, which had drawbridges in them, so the Mexicas could easily trap anyone in the city by merely raising the bridges. Stuck in the city, there was almost no access to food and water, which is why Cortés recognized that he had to get his forces out of there at night if they were to survive. His recognition that he needed to construct a portable bridge to use in their escape is an example of how quickly he thought of using technology to solve a military problem. He only failed when the Mexicas seized his bridge, leaving no way out for the others, who were killed.

While he was in Tlaxcala, he arrived at what he thought would be the perfect solution to defeat the Mexicas: he would construct sail-powered portable brigantines in pieces in Tlaxcala, carry them in pieces overland, then put them together on the outskirts of Tenochtitlan. These brigantines would be large flat barges that could carry nine horses, some cannon, and many soldiers. They would be covered with canopies, to protect the horses and men from arrows, spears, and darts, and would allow the Spaniards to come and go as they pleased even if the drawbridges were up.

Though the Mexicas had learned to modify their tactics and strategies to overcome the Spaniards' military advantages, they would not be prepared for the brigantines. This was an extremely creative idea, and Cortés recognized that the thirteen he built would be so key to the Spaniards' success that he would command the fleet.

When the brigantines were ready, Cortés began his reconquest of Mexico. He carried the brigantines to just outside Tenochtitlan, and then dug an enormous sealed canal, so he could secretly put the brigantines into Lake Texcoco. Once there, the siege of Tenochtitlan began.

The Mexicas immediately showed up in canoes to meet the brigantines, but they stayed far enough away that the Spaniards could not reach them. Then suddenly, the wind fortuitously shifted, effectively allowing the brigantines to mow down the canoes.

After the Mexicas had earlier forced the Spaniards to flee the city, they thought they were gone for good. They rebuilt the city and their temples and restored their markets. They also built a large wall around the city to fortify it, much like European cities were fortified.

The Spaniards, thus, began to use the cannons on the brigantines to blast away at the wall around the city. Recognizing they were facing fierce resistance, Cortés took command of the main brigantine fleet on the major causeway into the city, then allocated three each to the two commanders in charge of the other two causeways, Pedro de Alvarado and Gonzalo de Sandoval, the alaguacil mayor.

The Spaniards believed they would easily take the city, but found that it was much more difficult than they thought. Fierce fighting prevented them from taking the city directly. They would struggle all day to gain a causeway, meet fierce resistance within the city, then have to fall back at night, unable to even secure the causeway, which they would have to retake the next day. Nonetheless, they gained allies. Cortés also discovered that the horses and superior weapons were not of much use in street fighting, since both horses and soldiers were vulnerable to weapons thrown from the rooftops.

Eventually, Cortés decided that he would have to destroy the great city if he were to win. The aqueducts to the city were cut, and the brigantines swept the lake, preventing food and water from reaching the Mexicas. Slowly, over time, they began to starve to death. Moreover, as a result of their previous encounter with the Spaniards, they began to succumb to the plague, for they had no resistance to it. In fact, Cuitláhuac, who had been Moctezuma's successor, died of it six months after he took power, leaving the throne to Cuauhtémoc.

Sick, hungry, and suffering many casualties, the Mexicas continued their struggle. A bold Pedro de Alvarado, thinking he could capture the city's major marketplace, was trapped and suffered many casualties. Jubilant, the Mexicas then attacked another Spanish force and inflicted heavy casualties. Empowered by their victories, they visibly sacrificed fifty Spaniards and four horses, displaying their heads on a skull rack for all to see.

Cortés himself tried to take Tenochtitlan without success and experimented with a catapult to terrorize the population, but it failed to work. Cortés continued to beg the Mexicas to give up. Perhaps, as the Mexica sources suggest, the Mexicas had an omen to end the struggle and Cuauhtémoc surrendered or perhaps, as the Spanish sources indicate, the Spaniards captured him. In any case, rather than killing the emperor, Cortés imprisoned him, and the battle for Tenochtitlan was over.