The Chancas defeated Urco, Uira Cocha's oldest son and successor, and their army advanced toward Cuzco, subjugating the northern allies of the Incas. The victors came within sight of the capital, where meanwhile the energetic Yupanqui, Urco's younger brother, had gathered the whole force of the empire. The battle which decided the fate of Peru was fought on the heights above Cuzco; the Chancas were defeated, and fell back only to be pursued and overwhelmed by Yupanqui. He returned in triumph and was installed as emperor in place of his incompetent brother, assuming the title of Pachacutec, or "Reformer of the World." The Incas pressed their advantage relentlessly; all the tribes of the Chanca confederacy were subjugated; and Pachacutec's generals even extended their conquests north of Cerro de Pasco. The Incas had now conquered a practicable route to the Pacific, and the coast tribes about Lima soon also fell under their control. Pachacutec built a great military road from Cuzco north along the fertile plateau, through the smiling valley of Jauja, and down the short descent to the neighbourhood of Lima. Colonies were established at strategic points, and the new territory became so rapidly welded to the Inca system that, when the Spaniards arrived a hundred and fifty years later, they found the whole of central and southern Peru occupied by a homogeneous people, perfectly loyal to the Inca dynasty.
Pachacutec's successor, Tupac Yupanqui, proved even more successful than his father. The five hundred miles of rainless coast from Lima to the Ecuador border was inhabited by a mysterious race, in civilisation and origin entirely distinct from the Quichua-speaking mountain tribes to which the Incas belonged. Short rivers, rushing down from the Andes, each irrigated a portion of the desert, which only requires water to become extremely fertile. The irrigation works of this people were on a gigantic scale, one of their reservoirs having its lower end guarded by a dam eighty feet thick at the base. The valleys were cultivated to the highest degree of perfection, and filled with a swarming and industrious population housed in cities whose ruins still survive to attest the skill of their builders. Enervated by centuries of peace, the inhabitants had long confined their warlike operations to building defensive fortresses. Nevertheless, when Tupac advanced up the coast he met a desperate and prolonged resistance, until one after another the fortresses fell. The capital of the confederacy was laid in ruins and great numbers of the people were transported to distant provinces. Garrisons and Inca colonies were established and a military road was constructed along the coast. However, the country was really held only by force, and even in Spanish times Quichua had not displaced the Mochica tongue in half the northern coast valleys.