The pale skin guys were the catalyst. The Spanish rounded up everyone who hated the Aztecs (ie. everyone who interacted with them) and used that with their horses, guns and crossbows as shock troops to defeat them.

Future conquests (Inca, etc) were aided by the devastation of disease that wiped out stove populations.

Our situation isn’t so different. Look at how quickly the Soviet state degraded into near anarchy and what happened when the pieces stabilized.