It is believed that a combination of three factors led to the downhill of Tikal around 900 A.D. First, growing population strained the food supply and created a land shortage. Second, regional wars were common and they led to a social and cultural decline. Third, and probably most important, was drought. Scientists can now tell us that a very bad drought occurred at this time.
Tikal was not the only dominate culture to collapse during this time period. Teotihuacan also collapsed. By 400 AD Teotihuacan's population was already 200,000. It could not support this many people, and it was under attack by Chichimeca nomads. In addition, Oaxaca, Monte Alban, home of the Zspotec civilization and two other Mayan city-states, Palenque and Copan crumbled about this time. Drought and over use of the land may have also played a role in their demise.