Complex sentence: The battle was to determine which of the two alliances would have hegemony over Greece, but the death of Epaminondas and his intended successors and the impact of yet another defeat on the Spartans paved the way for Macedonian conquest led by Phillip II of Macedon.
Split into simpler sentences: The battle was to determine which of the two alliances would have hegemony over Greece. However, the death of Epaminondas and his intended successors coupled with the impact of yet another defeat on the Spartans weakened both alliances, and paved the way for Macedonian conquest led by Phillip II of Macedon.