Night of Sorrows War God Trilogy: Book Three

Cortés and his small army of Conquistadors enter Tenochtitlan, the island city of the Aztecs, as guests of the psychotic emperor Moctezuma who plans to trap them there and kill them all.
In a stunning coup, Cortés acts first, taking the emperor hostage and ruling the Aztecs through him. All of Mexico seems about to fall into his hands until a report comes from the coast of the arrival of a new force of Spaniards with more than three times his numbers, sent not to strengthen him but to attack him and wrest the conquest from him.
Faced with the choice of abject surrender or war with fellow Spaniards Cortés chooses war and marches out to do battle but, in so doing he fatally weakens his garrison in Tenochtitlan and throws open the doors of Hell.