Alexander left an army of ten or twelve thousand men with Antipater for the protection of Macedon. He organized another army of about thirty-five thousand to go with him. This was considered a very small army for such a vast undertaking. One or two hundred years before this time, Darius, a king of Persia, had invaded Greece with an army of five hundred thousand men, and yet he had been defeated and driven back, and now Alexander was undertaking to retaliate with a great deal less than one tenth part of the force.