Alexander became king after his father's death. He decided to carry on his father's plan to defeat Persia. He invaded Anatolia with 35,000 men and crushed Persia's defenses. The King of Persia, Darius III, then built up a huge army. Knowing he was outnumbered, Alexander focused on Persia's weak points and went straight for Darius. Alexander won and then went on to conquer the whole entire Persian empire. Alexander soon controlled all of Persia, Babylon, Susa, and Persepolis.