Complex sentence: The Spanish efforts were firmly resisted by the indigenous people, including the Pipil and their Mayan-speaking neighbors; they defeated the Spaniards and what was left of their Mexican Tlaxcala Indian allies, forcing them to withdraw to Guatemala.
Split into simpler sentences: The Spanish efforts were firmly resisted by the indigenous people, including the Pipil and their Mayan-speaking neighbors. They defeated the Spaniards and what was left of their Mexican Tlaxcala Indian allies, forcing them to withdraw to Guatemala.