Complex sentence: The battle itself had showed what a few free men, willing to do anything for victory against the invaders, could accomplish, and the defeat at Thermopylae had turned Leonidas and the men under his command into martyrs.
Split into simpler sentences: The battle itself had showed that a few free men were willing to do anything for victory against the invaders. The defeat at Thermopylae had turned Leonidas and the men under his command into martyrs.