The Aztec Empire was an empire continually at war, with an eye toward expansion. While there is no doubt that the gaining of further territory was a goal of the Aztecs, an equally important goal --and arguably the main, motivating goal -- for warring was to capture enemies that could be used for sacrifice.
For those who subscribe to the belief that indigenous populations of the New World were, by and large, innocent, peaceful, sheepish victims of the Spaniards, it has always made for uncomfortable reading when it comes to the bloodthirsty sacrificial exploits of the Aztecs.
An Inescapable Fate
Let us say that you are a captured enemy of the Aztecs. What sort of fate might you expect? First of all, it was not uncommon to be a Johnny-on-the-spot sacrifice right there on the battlefield, after which you would be cooked and eaten. The rest of your unhappy comrades would be herded back to Tenochtitlán, the Aztec capital, or to some lesser population center.
The next step in your tragic fate would be to be fattened up while waiting in a wooden cage. The fatter you are, the more of you there is to eat when your appointed time comes. Next, at a signal from a priest, you would be led up the side of a pyramidal temple. The altar before you and the river of blood running down the steps of the temple would leave no doubt in your mind as to your ultimate fate. Hysterics at this point will do you little good as you are secured face up on the altar. The priest, the last person upon this earth that you will look at, wears a feathered cap and is covered in ceremonial black body paint. The tired priest has had a long day, but the downward thrust of his knife is purposeful and unhesitating, as if he has performed this ritual act a thousand or more times before. The knife is plunged into your body. There is a cold shock. Then the cutting begins. Your heart is cut out and grasped by the hand of the priest, who then lifts it up high for all to see. The heart is at the center of all such ceremonies and is used to appease the sun god, and is now formally offered to the sun god.
Your value as a victim of the Aztecs does not end here, however. Your body is kicked down the steps of the pyramid, and there received by men at the bottom who will take it away to be butchered. Your head will go on one of the skull racks that line the central plazas, perhaps to be eaten later by the priests at their discretion, but probably not before the skin is stripped off and made into leather. As for the rest of you, at least three of your limbs will go to the person who bravely captured you. They are his by right of capture. However, this is assuming the capture was done unassisted; otherwise others will be on the scene to lay claim to your limbs. Your limbs will then typically be used as the main ingredient of a stew, along with pepper, tomatoes and so on, and served at a banquet held in the honor of the person who captured you. As for your torso, if you are in Tenochtitlán, then you're bound to wind up in the local zoo as a meal for the animals.
Conclusion
While abhorent to outsiders and particularly to the Spaniards who were eyewitnesses to these displays of human sacrifice and cannibalism (some of whom ended up in the pot themselves), it was not without a practical side: shortages in game, periodic droughts and famine and overpopulation were all motivating factors to the practice of cannibalism.
The need for sacrifices would ultimately be a very relevant contributor to their undoing, since it would prompt the Aztecs' neighbors to more readily ally themselves with the invading Spaniards against them.
