Complex sentence: Upon seeing the disposition of Pompey's army Caesar grew discomforted, and further thinned his third line in order to form a fourth line on his right: this to counter the onslaught of the enemy cavalry, which he knew his numerically inferior cavalry could not withstand.
Split into simpler sentences: Upon seeing the dispositions of Pompey's army, Caesar grew discomforted and further thinned his third line in order to form a fourth line on his right. This to counter the onslaught of the enemy cavalry, which he knew his numerically inferior cavalry could not withstand.