[throwup238]

The Tlaxcalans made the mistake of engaging the conquistadors where their cavalry was most effective and allowed them to fortify themselves at Tzompachtepetl, at which point Spanish armor provided to be too effective for Tlaxcalan tactics (send wave after wave of troops instead of overwhelming them). Eventually they just kind of gave in and weren't really defeated in great number before allying against the Aztecs.

In their very first skirmish at the edge of Tlaxcalan territory where it was mountainous and undeveloped, a small scout troop took down two horses out of the sixteen that Cortex had with him. Spanish tactics and discipline were enough to overcome large numbers only when they controlled where the battles happened. If Montezuma had used guerilla tactics during the subsequent trip, it would have been far more effective (he had plenty of spies reporting on their movements so I don't think the logistics would have been that difficult). Cortes even lengthened the journey in part to minimize how much time they'd spend in terrain that favored those tactics.

Maybe "obsessed" is the wrong word but Montezuma was influenced by legends of Quetzalcoatl as a white-skinned, bearded man who opposed human sacrifices and was reluctant to just go out and slaughter the conquistadors. Their religion was definitely an influence in the whole affair beyond just the whole human sacrifice bit.