Teotihuacan: The City of the Gods does mention what seems to be a stronger military presence in the city, but it's attributed to the later period, close to the decline of the city. What's interesting to note is that Teotihuacan, through most of its existence, had a very high concentration of obsidian workshops - during the Tlamimilopa Phase there were more than 400. While not all of these were solely dedicated to the production of knives and weapon heads, there were at least a hundred that were. It's hard to believe that the advent of those particular workshops could be attributed to trade and trade only. And if they sprang from necessity, that would designate some type of military force early in the city's development - with the growth of the city, even through agriculture and trade, would they really have decreased their military capabilities? I wouldn't think that likely - especially considering they'd have a heavier influx of foreigners visiting the city.