1. The Mexicas' (correct name of the Aztecs) religion, particularly the belief in feeding the Sun/War God, Hutzilopochtli, required human blood in order to stay alive and continue its journey across the skies, had the effect of making the capture of enemy prisoners a priority in war, instead of merely disabling or eliminating them (a far easier task for it exposes warriors less to wounds and fatal injuries). In battle against the Conquistadors, the Mexica practice of seeking out prisoners of war in battle turned out to be a distinct handicap readily exploited by the Spaniards.
2. The Incas managed to maintain control of the vast territories under their control by means of a an excellent communication system consisting of paved roads across the empire, the establishment of fortified military bases garrisoned by Incan warriors and the efficient use of messengers and their distinct record system known as quipu.
3. Both the Mexicas and the Incas started out as small, poor yet resourceful and ambitious peoples who were able to rebel against foreign domination and expand their domains through the creation and maintenance of highly efficient, aggressive and disciplined armies.
Mexicas and Incans differed in the use of their armies after consolidating their empires: the Mexicas waged wars yearly with the purpose of capturing as many prisoners of war as possible in order to offer their lives as sacrifice to their Sun/War God Huitzilopochtli. Conversely, the Incans used their army to garrison fortified posts strategically located throughout their empire in order to avert any chance of rebellion. Also, the way political and military power was exerted was different in each civilization. The Mexicas were ruled by about half a dozen lords called "tlatoanis" who in turn appointed an overlord called "great tlatoani," so their government system was an elective monarchy of sorts. On the other hand, the emperor of the Incas called "Sapa Inca" inherited the power from his father and would usually murder his brothers as they were his rivals for the supreme power. Thus, the Incas had a hereditary monarchy, a more traditional kind of government.