Using historical records, logic, and historical generalizations, we can attempt to hypothesize about the nature of the spear as it relates to tactics and formations in the Aztec world.
The spear was arguably the first true weapon invented by man. Early spears were initially constructed of a knapped stone point affixed to a stick or pole. In Mesoamerica, the spear had evolved into a quasi-halberd by the time the Aztecs established their empire. As such, the Tepoztopilli could be thrust and swung at an enemy.
The tactics and formations used by the Aztec military were, like any other culture, influenced by the design and purpose of the weapons that they used. For example, sword or club wielding infantry formations often had to be loose to allow soldiers room to outmaneuver their opponent, whereas spear infantry formations were generally densely packed in order to maximize the number of combatants on the front line.
Lets say, hypothetically, two armies of spear-wielding soldiers were set to engage each other in combat. Both sides would obviously attempt to get an advantage over the other before the battle began. If one army (army A) brought spears that were, say, 2 meters in length while the other (army B) brought spears that were 3 meters in length, the latter party would succeed if the armies were relatively the same in size and training.
Army B is capable of engaging their enemy from farther away, making them more effective in battle. However, a longer spear is often less effective in single combat. If an opponent with a shorter spear is able to get past his enemy’s longer one, he will be more effective because he can close the distance and maintain better control of his weapon.
To circumvent this, army B must pack its soldiers closer together. If the soldiers are standing closer to each other, their spears will form a “spearwall”, making it very difficult for the enemy to close in and engage the formation. Based on these simplistic circumstances, army B would be successful in most engagements. However, if a spear is too long, it is less likely be paired with a shield because it often requires both hands to use. This could be circumvented by affixing the shield to the combatant’s forearm, but this limited the shield’s protective capability.
The Aztec army drafted massive numbers of commoners into their army and many of these commoner soldiers were dedicated slingers or archers. A tightly packed, unshielded unit formation is a perfect target for projectile fire.
If army A decided to dedicate half of its forces to projectile fire, why should they charge the enemy army at all? Army B would be hard-pressed to chase down more agile, less burdened archer units of their enemies. Army A can simply fire volleys into army B’s formations, fall back until their enemies stop pursuing, and then continue firing. Because of this, the spear units of army B must have some kind of shield.
Because of these factors, and the subsequent “innovation/arms race” that they bring, a compromise must be made. Spears had to be made as long as possible in order to keep the enemy at a distance, but short enough to ensure that they weren’t unwieldy and a shield could more easily be used with them. This enabled the unit to protect itself from both charging infantry and projectile fire, at least to some extent. In addition, unit formations could not be too tight, else the soldiers within them could not properly maneuver during combat and they could potentially be targeted by projectile fire. However, they had to be dense enough to ensure that charging infantry could not rush through the gaps in their line.
The nature of the spear likely fluctuated to some extent during the time of the Aztecs. These fluctuations in length and function were in response to specific regional circumstances as well as larger shifts in warfare as a whole.