Complex sentence: This was the pattern of Spanish policy to seek peace with Indians on their northern frontier that would continue for another three centuries to confront later threats from hostile Indians such as Apaches and Comanches.
Split into simpler sentences: This was the pattern of Spanish policy to seek peace with Indians on their northern frontier. It was a policy that would continue for another three centuries to confront later threats from hostile Indians such as Apaches and Comanches.