candidate: Restall argues that the factors behind the success of the conquistadors were mostly the devastating effect of European diseases for which the Indians had no resistance , the disunity between indigenous groups some of which allied with the Spaniards early , the technological advantage of the steel sword , native battle practices that were not upheld by the Spaniards — such as killing non-combatants and civilians , and most importantly the fact that the Indians were fighting on their own ground with their families and fields to care for , which made them quicker to compromise .