This contributed to the downfall because Civil war between the two sides weakened the military. The two sons were able to reign over their separate halves peacefully for nearly five years before Huascar Inca thought differently. He secured power in Cuzco and then went on to have Atahualpa, his brother, arrested. Atahualpa was able to escape his imprisonment with his wife's help and he began to make sure he had support from Huayna Capac's best generals. Atahualpa rebelled strongly against his brother and in the end he won the civil war, he even imprisoned Huascar near the end of the war.
