Throughout history, warfare has played an important role in the development of many cultures around the world, and Mesoamerica is no exception. As J.M. Francis and T.M. Leonard noted, "The history of pre-Columbian Mesoamerica was one of violence, though no more so than that of any other region of the ancient world. It was a universe of shifting alliances and mutual antagonisms, in which increasingly strong political entities forged themselves and then broke apart." Of course, the history of warfare in Mesoamerica is a long one, tracing its origins back to the Preclassic period with the Olmecs, who were the first group to expand their influence. There is clear evidence they had a military development, but it seems to have been mainly focused on protecting their trade networks instead of on conquest. After the Olmecs, Teotihuacan rose as the first main center of military expansion during the Classic period, extending its influence across Mesoamerican territory. After its collapse, the vacuum of power created an unstable period, and new expanding polities emerged, including Cacaxtla, Xochicalco, and Tula. They became important regional centers that took control of most of central Mesoamerica through the use of military interventions. After their decline, during the Postclassic period, the Aztecs eventually emerged as the dominant empire in Mesoamerica and continued expanding their control and influence until the arrival of the Spanish in the 16th century. Perhaps not surprisingly, there are many misconceptions about Prehispanic warfare, so it is important to keep in mind that military conflicts in Mesoamerica were greatly diverse and cannot be summarized by simply analyzing Aztec militarism.