What convinced Inca armies to retreat in their war against the Spanish in 1533?

The Spanish conquest of the Inca Empire was one of the most important campaigns in the Spanish colonization of the Americas. After years of preliminary exploration and military skirmishes, 180 Spanish soldiers under conquistador Francisco Pizarro, his brothers, and their native allies captured the Sapa Inca Atahualpa in the 1532 Battle of Cajamarca. It was the first step in a long campaign that took decades of fighting but ended in Spanish victory in 1572 and colonization of the region as the Viceroyalty of Peru. The conquest of the Inca Empire led to spin-off campaigns into present-day Chile and Colombia, as well as expeditions towards the Amazon Basin. When the Spanish arrived at the borders of the Inca Empire in 1528, it spanned a considerable area; by far the largest of the four grand pre-Columbian civilizations. Extending southward from the Ancomayo, which is now known as the Patía River, in southern present-day Colombia to the Maule River in what would later be known as Chile, and eastward from the Pacific Ocean to the edge of the Amazonian jungles, the empire covered some of the most mountainous terrain on Earth. In less than a century, the Inca had expanded their empire from about 400,000 km² (155,000 sq mi) in 1448, to 1,800,000 km² (690,000 sq mi) in 1528, just before the arrival of the Spanish. This vast area of land varied greatly in cultures and in climate. Because of the diverse cultures and geography, the Inca allowed many areas of the empire to be governed under the control of local leaders, who were watched and monitored by Inca officials. However, under the administrative mechanisms established by the Inca, all parts of the empire answered to, and were ultimately under the direct control of, the Emperor.[1] Scholars estimate that the population of the Inca Empire numbered more than 16,000,000.