Teotihuacan was the first of the major empires of what is now Mexico. It had begun to have major power by about 200 BC. Scholars believe that its rise was due to two things. First, it enjoyed a near monopoly on obisidian, which was an important stone. Second, it was a major religious center, especially for Quetzalcoatl worship.
Scholars believe that the empire started to decline because of problems with agriculture. This allowed tribes from the north to invade and weaken the empire further. The empire fell by about 600 AD.
Teotihuacan was a large empire that stretched from Guatemala to Texas. The only other civilization that could even come close to matching its glory was the Roman Empire. Violent and ritualistic sacrifices were carried out in the city. The cities population grew to over 200,000 people.
The collapse of the great city is uncertain but has been pieced together by scholars who believe that it was invaded by enemies. The Toltecs were believed to have burned many of the primary structures. In addition there is the belief that droughts caused the crops to die and the people of the area to suffer famine and malnutrition. In addition, there may have also been unrest politically within the city.