Scholars had originally thought that invaders attacked the city in the 7th or 8th century, sacking and burning it.
More recent evidence, however, seems to indicate that the burning was limited to the structures and dwellings associated primarily with the ruling class.
Some think this suggests that the burning was from an internal uprising.
They say the invasion theory is flawed, because early archaeological work on the city was focused exclusively on the palaces and temples, places used by the upper classes.
Because all of these sites showed burning, archaeologists concluded that the whole city was burned.
Instead, it is now known that the destruction was centered on major civic structures along the Avenue of the Dead.
The sculptures inside palatial structures, such as Xalla, were shattered.
No traces of foreign invasion are visible at the site.

Evidence for population decline beginning around the 6th century lends some support to the internal unrest hypothesis.
The decline of Teotihuacan has been correlated to lengthy droughts related to the climate changes of 535–536.
This theory of ecological decline is supported by archaeological remains that show a rise in the percentage of juvenile skeletons with evidence of malnutrition during the 6th century, which is why there is different evidence that helps indicate that famine is most likely one of the more possible reasons for the decline of Teotihuacan.
The majority of their food came from agriculture: They grew things such as maize, beans, amaranth, green tomatoes (tomatillos?),
and pumpkins, but their harvest was not nearly sufficient to feed a population as big as it is believed have lived in Teotihuacan.
This finding does not conflict with either of the above theories, since both increased warfare and internal unrest can also be effects of a general period of drought and famine.
Other nearby centers, such as Cholula, Xochicalco, and Cacaxtla, competed to fill the power void left by Teotihuacan's decline.
They may have aligned themselves against Teotihuacan to reduce its influence and power.
The art and architecture at these sites emulate Teotihuacan forms, but also demonstrate an eclectic mix of motifs and iconography from other parts of Mesoamerica, particularly the Maya region.

The sudden destruction of Teotihuacan was common for Mesoamerican city-states of the Classic and Epi-Classic period.
Many Maya states suffered similar fates in the coming centuries, a series of events often referred to as the Classic Maya collapse.
Nearby, in the Morelos valley, Xochicalco was sacked and burned in 900 and Tula met a similar fate around 1150.

There is a theory that the collapse of Teotihuacan was caused by the devastation of its agriculture by the 535 CE eruption of the Ilopango volcano in El Salvador.
Scholars had originally thought that invaders attacked the city in the 7th or 8th century, sacking and burning it. More recent evidence, however, seems to indicate that the burning was limited to the structures and dwellings associated primarily with the ruling class. Some think this suggests that the burning was from an internal uprising. They say the invasion theory is flawed, because early archaeological work on the city was focused exclusively on the palaces and temples, places used by the upper classes. Because all of these sites showed burning, archaeologists concluded that the whole city was burned. Instead, it is now known that the destruction was centered on major civic structures along the Avenue of the Dead. The sculptures inside palatial structures, such as Xalla, were shattered. No traces of foreign invasion are visible at the site.
Evidence for population decline beginning around the 6th century lends some support to the internal unrest hypothesis. The decline of Teotihuacan has been correlated to lengthy droughts related to the climate changes of 535–536. This theory of ecological decline is supported by archaeological remains that show a rise in the percentage of juvenile skeletons with evidence of malnutrition during the 6th century, which is why there is different evidence that helps indicate that famine is most likely one of the more possible reasons for the decline of Teotihuacan. The majority of their food came from agriculture: They grew things such as maize, beans, amaranth, green tomatoes (tomatillos?), and pumpkins, but their harvest was not nearly sufficient to feed a population as big as it is believed have lived in Teotihuacan. This finding does not conflict with either of the above theories, since both increased warfare and internal unrest can also be effects of a general period of drought and famine. Other nearby centers, such as Cholula, Xochicalco, and Cacaxtla, competed to fill the power void left by Teotihuacan's decline. They may have aligned themselves against Teotihuacan to reduce its influence and power. The art and architecture at these sites emulate Teotihuacan forms, but also demonstrate an eclectic mix of motifs and iconography from other parts of Mesoamerica, particularly the Maya region.
The sudden destruction of Teotihuacan was common for Mesoamerican city-states of the Classic and Epi-Classic period. Many Maya states suffered similar fates in the coming centuries, a series of events often referred to as the Classic Maya collapse. Nearby, in the Morelos valley, Xochicalco was sacked and burned in 900 and Tula met a similar fate around 1150.
There is a theory that the collapse of Teotihuacan was caused by the devastation of its agriculture by the 535 CE eruption of the Ilopango volcano in El Salvador.
Teotihuacan