Complex sentence: Darius was building up a massive army, drawing men from all parts of his empire, more than 250,000 soldiers (including more than 42,000 cavalry) against approximately 47,000 Macedonian soldiers (including around 8,000 cavalry).
Split into simpler sentences: Darius was building up a massive army, drawing men from all parts of his empire. He gathered more than 250,000 soldiers (including more than 42,000 cavalry) against approximately 47,000 Macedonian soldiers (including around 8,000 cavalry).