Alexander, like his father, was obsessed by the idea of leading an expedition into Asia. Alexander started his march with an allied Greek army that proved to be one of the most impressive in history. In the spring of 334 bc Alexander crossed the Hellespont heading for Asia, leaving Antipater, the general and old friend of his father, as his deputy in Europe. Alexander's main opponent there was King Darius III, the last in the line of the so-called Great Kings of Persia, an empire started with Cyrus the Great. A major by-product of Alexander's victory at the Granicus was that many prominent Persian commanders were killed. Alexander's network of informers had apprised him of the inexperience of some of the troops in the battle formation of Darius, however, so the Greek king rushed his Companion Cavalry towards those inexperienced troops who could not stand up to the Macedonian assault.