After Tlacaelel inculcated the idea that Huitzilopochtli- the Sun had to be fed with the blood of human sacrifices; war became a cultural institution of primary importance in Aztec life, since war was the means of obtaining victims to appease the god's insatiable hunger. Regardless of the ostensible purpose of a military campaign-to conquer new territory, punish a rebellious vassal state, or repel an aggressor-the Aztec warriors never forgot that their first duty was to take captives to be sacrificed. This religious conception of warfare motivated the expansion of the Aztec empire, but it also contributed to its destruction by the Spaniards. On several occasions the Aztecs probably could have wiped out the Spaniards to the last man-their best chance of all was on the Night of Sorrows-but the ceremonial elements in their attitude toward war prevented them from taking full advantage of their opportunities.
As in other cities in central Mexico, military training in Tenochtitlan began during early youth. The army was made up of squads of twenty men, which were combined to form larger units of about four hundred, under a tiachcauh who came from the same clan as the warriors he commanded. The more important leaders were usually Eagle or Jaguar Knights, with such titles as tlacatecatl (chief of men) and tlacochcalcatl (chief of the house of arrows).
The most important offensive weapon of the Aztecs was the Macana, a sort of paddle-shaped wooden club edged with sharp bits of obsidian. It was so awesomely effective that on more than one occasion during the Conquest warriors beheaded Spanish horses at a single stroke. Other widely used arms were the atlati, or spear thrower, bows and arrows of different sizes, blowguns and a variety of spears and lances, most of them with obsidian points. The defensive weapons were shields made of wood or woven fibers-often elaborately painted and adorned with feathers-and quilted cotton armor. Some of the warriors also wore various types of masks and headdresses to show that they were Eagle or Jaguar Knights or belonged to the higher military ranks.
A war or battle always commenced with a certain ritual: shields, arrows and cloaks of a special kind were sent to the enemy leaders as a formal declaration that they would soon be attacked. This explains the Aztecs' surprise when the Spaniards, their guests, suddenly turned on them without any apparent motive and-more important-without the customary ritual warning.