The first thing that Montezuma I concentrated on after his coronation in 1440 was to assert Aztec reign by consolidating claims on towns already conquered by Itzcoatl in the Valley of Mexico. Having received assurances of their submission, he then used the rebuilding of the Great Pyramid as pretext for soliciting help from additional cities. His next step was to send an expedition to secure towns in Morelos and Guerrero, which had for the most part already been tributepayers in the old Tepanec domain.

During the first 18 years of his reign he had effectively strengthened the cohesion of the alliance with the rulers of Tetzcoco and Tlacopan, an established firm foundations for an ambitious expansion policy. He defeated Chalco and its allies and opened the way for a series of extraordinary campaigns that were to take the army far from the mountains of the Valley of Mexico.

The Aztec first moved on the Huaztec region of north central Veracruz. Once the Huaztecs were successfully defeated by the Aztecs. This victory forced all enemies to recognize that a much larger military enterprise could be organized and launched. The Aztec's next move was into Mixtec territory. The motives of this move were to obtain tribute.
In Tenochtitlan, the first step in the campaign against Coixtlahuaca was to send out the call for warriors to the four wards of the city and to the allied rulers and client chieftains. Warriors assembled in their own districts and towns, and moved towards areas of concentration. The larger formations then began to depart at intervals, in an army totaling about 2000,000 men , supported by 1000,000 porters. The assembled ranks confronted each other across open ground. The Aztec force broke through the defenders' ranks and a battle developed as the Mixtecs fell back through the town. The Aztec warriors were urged on by their captains to the principal pyramid. Sensing victory they broke through again and ran up the stairs to set fire to the roof of the temple.

Under Montezuma I the Aztecs won a decisive series of conquests that set a course of military expansion that was to dominate Aztec policy until the Spaniards arrived. By the time of Montezuma's death in 1469 and Netzahualcoyotl's death in 1472, the lands paying tribute to the Aztecs reached an imaginable wealth from tropical lowlands and the upland plateaus now poured into the imperial cities on a regular basis.