What mountain range forms a natural border between the carolingian empire and Spain

The battle of sabine pass is important because? a) it was a stalemate between union and confederate armies. b) it resulted in the largest confederate casualty in the war. c) it marks the failed attempt of the union army to invade the confederate state of texas. d) it was won by the union soldiers and marked a turning point in the western front of the war.