One wonders what makes this area in northern Iraq one of history's major fault lines -- places marked over and over again by historic eruptions that literally change the course of time -- places where two immutable forces come together, forcing one to give only after the greatest of struggles. The Persian army had massed near Babylon. If Darius had stayed here, Alexander would have been drawn southward into a battle site of Darius' own choosing. Instead the Persian leader pushed his troops 320 kilometers northward to meet the advancing Alexandrian army in the small town of Gaugamela, on one of the most active fault lines in history.

• Michael Wood