Why did Inca decline?

The Incas were a well civilized race, however they still had their downfalls. The man who conquered this fierce tribe was Pizarro. He didn't just conquer them with pure force though. Before Pizarro arrived a great civil war had occurred, leaving the Inca military weak, which made it easy for Pizarro to destroy them. Pizarro had taken Atahualpa, the Incan leader, captive. The incas bribed Pizarro with a room filled with gold for Atahualpa, however once Pizarro received it he still ordered that Atahualpa to be killed. It took Pizarro approximately 40 more years until the last Inca was killed, his name was Tupac Amaru, Pizarro ordered that his body be dismembered to stop any further Inca resistance.