In 738 , Quiriguá , a vassal of Copán , Tikal 's key ally in the south , switched allegiance to Calakmul , defeated Copán and gained its own independence . It appears that this was a conscious effort on the part of Calakmul to bring about the collapse of Tikal 's southern allies . This upset the balance of power in the southern Maya area and lead to a steady decline in the fortunes of Copán .