Complex sentence: Victory through a cunning alliance is impossible because game rules dictate that there can only be "one" winner at the end of the game; that is why the two former allies need to go to war in order to claim the victory.
Split into simpler sentences: Victory through a cunning alliance is impossible because game rules dictate that there can only be "one" winner at the end of the game. That is why the two former allies need to go to war in order to claim the victory.