How did the enemies of the Aztec help the Spanish to defeat the Aztec?

The Aztec Empire, which flourished between 1420 and 1520 approximately, dominated many of the neighboring populations. Some were allies, but many remained resentful enemies, even though they were subjugated. Some, like the Tlaxcalan, remained independent, despite the fact that the Aztec tried to conquer them many times.
When Cortes arrived in Mexico in 1519, he had to pass through the lands of many of the Aztec enemies, such as the Tlaxcalan. Cortes managed to make allies of the Tlaxcalan and of other populations by promising them freedom from the Aztec and giving them gifts. He understood that he needed the support and military strength of other populations who knew the land and the Aztec better. He also needed a safe base in which to rest and recover after battles.
With the help of these neighboring tribes, the Spanish easily defeated the Aztec. Cortes quickly understood that the internal resentment which many tribes harbored against the Aztec could be useful to the Spanish, and he used it to their advantage.