Although numerous battles were fought between the Aztec Empire and the Spanish-led coalition, which was itself composed primarily of indigenous (mostly Tlaxcaltec) personnel, it was the siege of Tenochtitlan—its outcome probably largely determined by the effects of a smallpox epidemic (which devastated the Aztec population and dealt a severe blow to the Aztec leadership while leaving an immune Spanish leadership intact)—that directly led to the downfall of the Aztec civilization and marked the end of the first phase of the Spanish conquest of the Aztec Empire.