Final answer:
Itzcoatl is the Aztec ruler credited with uniting Tlacopan, Texcoco, and Tenochtitlan to form the Triple Alliance in 1428, marking the rise of the Aztec Empire.
Step-by-step explanation:
The Aztec ruler credited with uniting Tlacopan, Texcoco, and Tenochtitlan to form the Triple Alliance, which increased Aztec military power, is Itzcoatl. This alliance was formed in 1428 and laid the foundation for what became known as the Aztec Empire.
Itzcoatl's leadership was pivotal in this political and military union, enabling the Aztecs to expand their territory and influence across central Mexico through a combination of military conquest and strategic alliances.