Final answer:
Pachacuti was the Sapa Inca who initiated the expansion of the Inca Empire, embarking on a series of conquests that began the empire's growth throughout the continent.
Step-by-step explanation:
The Sapa Inca who started the expansion of the Inca Empire was Pachacuti, reigning from 1438 to 1471. Pachacuti initiated the empire's growth by conquering territories that are part of modern-day Ecuador, Bolivia, and Peru, including the significant victory over the Chimor, a powerful kingdom north of Cuzco. The leap to imperial expansion is also attributed to a legend involving Pachacuti and a military challenge from the rival Chanka army, which ultimately led to his rise to power and the beginning of the Inca Empire's era of conquest and expansion. Furthermore, it was under Pachacuti's leadership that the foundation for subsequent military successes under his successors, Thumpa and Wayna Qhapaq, was established, allowing the empire to grow even further into various parts of South America.