What events led to the creation of the Aztec empire?

Migration into Mesoamerica, then after constantly being attacked by the more powerful civilizations native to the region, they settled in Lake Texcoco. They built their capital Tenochtitlan in the middle of the lake and began their rise to power. At first they were actually under the rule of neighboring city-state known as Azcapotzalco, and wasn't until the Aztec ruler Ixcoatl made a few alliances that they became truly independent. Then, they put the city-state of Azcapotzalco under their rule and gradually they began to conquer other tribes and city-states afterward and they created an empire.