The empires of the Americas are interesting and worthwhile case studies within the greater context of imperialism.
The Mayan empire largely relied on loose ties between independent city-states. While each city-state retained their unique cultures, the result were many wars between these entities.
In juxtaposition to the Mayans, the Incas relied on force over soft diplomacy. The Incas subjugated independent tribes and groups of people into their ranks. When faced with the choice of starting a war against the Incas or peacefully joining the empire, many chose peace. As payment for joining the empire, the subjects contributed labor service which resulted in an impressive system of roads and bridges, additional farming lands in mountainside terraces, and ritual centers.
The Incans had a network of storehouses complete with food, clothes, and weapons to either help their subjects in a time of crisis or squash an uprising.
There were two additional important aspects that solidified the power of the empire and showed important tactics of imperialism: forced adoption of language and religion. The Inca emperor was viewed as the son of the sun god, which means the subjects were forced to align with this divine ruler. Over time, this meant that different groups lost their individual cultures as the Incas succeeded in making a more homogenous empire.