Complex sentence: Between the 10th and 13th century it was integral to the Cambodian Angkor, and their empire went into decline between 15th and 17th century when it was annexed under the Lan Xang kingdom.
Split into simpler sentences: Between the 10th and 13th century it was integral to the Cambodian Angkor. The Angkor empire went into decline between 15th and 17th century when it was annexed under the Lan Xang kingdom.