Complex sentence: The first day's battle was long and bloody, terminating without any decisive advantage to either side; but although the Persians were tactically successfull, on the second day Astyages, making skilful use of his superior numbers, gained a great strategic victory.
Split into simpler sentences: The first day's battle was long and bloody, terminating without any decisive advantage to either side. On the second day, although the Persians were tactically successfull, Astyages, making skilful use of his superior numbers, gained a great strategic victory.