Excerpts from Books and Wikipedia
Pachacuti ruled from 1438 to 1471. The victory was the beginning of Incan expansion with the population reaching as high as ten million. [Furtado: 1001 Days] "Prince Cusi Yupanqui becomes the first Inca emperor. Inca civilization begins expanding and the Inca Empire is born. [Wikipedia: Timeline of Middle Ages] "Pachacuti Inca Yupanqui (Quechua: Pachakutiq Inka Yupanki) was the ninth Sapa Inca (1418–1471/1472) of the Kingdom of Cusco which he transformed into the Inca Empire (Quechua: Tawantinsuyu). Most archaeologists now believe that the famous Inca site of Machu Picchu was built as an estate for Pachacuti." [Wikipedia: Pachacuti]