You also had the Mongols, arguably the most successful nation in history, particularly with conflict. They relied heavily on spies and would often observe for years before the Mongols struck. Ever hear of the Khwarazmian empire? One of the largest middle eastern empires? Worth reading about. The Mongol's success depended largely on spies. Success in this instance means "wiping the empire from the face of the earth", which is obviously subjective.