Pizarro and his forces attacked the Inca and captured their Emperor, Atahualpa, because the Inca Empire was weak because of a civil war.
Atahualpa was the last Inca Emperor.
The conquest of the Inca Empire originated on November 16, 1532 when an Inca army ran into the Spanish conquistadors led by Francisco Pizarro in Cajamarca, shortly following the end of the civil war between the two heirs to the Inca throne, Huascar and Atahualpa. Consequently, Atahualpa was taken prisoner by the Spaniards and months later executed on July 26, 1533.