What caused the Classic Maya collapse: drought, war or too many people?

Nobody knows for sure, and that is the honest answer. The southern lowland Maya cities were abandoned between the 7th and 9th centuries, and more than 80 theories have been identified, but there is no universally accepted one. The likeliest answer is a combination of causes: endemic warfare, overpopulation resulting in severe environmental degradation, and drought.

The pattern is clear even if the cause is not. Dated monuments rose from about ten a year in 514 to forty by about 750, then fell to ten by 800 and to zero by 900. Drought has gained momentum in the 21st century as the leading explanation. A 1995 record from Lake Chichancanab, by Hodell, Curtis and Brenner, showed an intense, protracted drought in the 9th century AD, and a 2018 study in Science, the most robust estimate to date, found that annual rainfall fell by 41 to 54 percent, with peaks of up to 70 percent. Critics, including David Webster, note that much of the drought evidence comes from northern Yucatán, not the southern lowlands.

Foreign invasion is a weaker idea: most Mayanists do not believe it was the main cause, because no military defeat can explain such a protracted process. At Tikal, as the area reached peak population it suffered deforestation, soil erosion and nutrient loss, and there is not much evidence the city was directly affected by warfare.

Even the word "collapse" is contested: it did not end Maya civilization, and northern Yucatán prospered. At Caracol the rulers were blamed and eventually moved away, but the common people stayed.