Why Was the Horse the Conquistadors' Most Important Weapon?

Si Sheppard says the horse was the vital weapon once the conquistadors left the coast, more decisive than the other animals they brought.

Sail power let them land where they chose. Inland, a horse could break a native formation that lacked the weapons, training, or discipline to stand up to a cavalry charge, and it could carry the Spaniards away faster than anyone could chase them. The Inca already had a pack animal, the llama, and campaigning armies trailed baggage trains of llamas for miles. The Aztecs had no equivalent. Their only domestic animals were the turkey and the chihuahua. The Spaniards also brought armored war mastiffs trained to attack men, which terrified people who had never seen them. The horse was still the asset that decided the fight.

The Tlaxcalans, the last major people in the region still holding off the Aztecs, saw those assets and treated an alliance as a way out of a closing trap. Flower wars were turning into conquest, and they had been cut off from the coast and from salt.