Alexander had no obvious or legitimate heir his son Alexander IV by Roxane being born after Alexander's death. Balkan campaign
Before crossing to Asia Alexander wanted to safeguard his northern borders. He had a great desire for knowledge a love for philosophy and was an avid reader.
Alexander broke the power of Persia in a series of decisive battles most notably the battles of Issus and Gaugamela. At that point his empire stretched from the Adriatic Sea to the Indus River. By the age of thirty he had created one of the largest empires of the ancient world stretching from the Ionian Sea to the Himalayas.