Since 500 BC the mainland Greeks had been threatened by the Achaemenid Persian Empire. They had suffered major invasions but subsequent attempts to take the offensive had been thwarted. With Alexander the Great's invasion the rules changed. In Macedonia a new model army had been developed, taking the traditional hoplite heavy infantry in a new evolutionary direction and similarly transforming the heavy cavalry. These developments neutralized the Persians' own efforts to modernize their troops, tactics and equipment. Despite the inclusion of a state-of-the-art siege train, the structure of the reformed Macedonian army allowed an unprecedented operational tempo. Manousos Kambouris' detailed analysis explains that it was Alexander's intelligent use of these forces, that allowed him to dictate the course of the campaign. His excellent strategic and operational decision-making, based on an intimate knowledge of geography and logistics, along with well-timed movements and clever feints, allowed him to choose his battles, which he then won by tactical brilliance and guts. The author does not neglect to assess the Persian capabilities and decision making, concluding that Darius III was not as inept as often thought. Indeed, he may have been the most militarily capable King of Kings but it was his misfortune to be pitted against the genius of Alexander, the great avenger.