Complex sentence: They were divided between the 5th army in the west and the 10th army in the east and thus spread out from the Tunisian border in western Libya to Sidi Barrani in Egypt.
Split into simpler sentences: They were divided between the 5th army in the west and the 10th army in the east. Hence, they remained spread out from the Tunisian border in western Libya to Sidi Barrani in Egypt.