Why did Alexander the Great's empire fall?

Why did Alexander the Great's empire fall? (click headline for original source) It’s a really big mess. Dramatic though. The following was spread out across more than a dozen Wikipedia articles, and I’ve drawn together what I can to form this comprehensive and long summary. In short: Lots of fighting between generals resulted in three of them claiming empires in Asia, Greece and Egypt. All of these later fell to the expanding Romans. In long: Alexander , the military genius who conquered all the land from Greece to India in a decade without ever losing a battle, suddenly died in June 323 BCE. He was only 32 and had no children (yet - his wife Roxana would give birth after his death), and had made plans to invade Arabia and set up his capital at Babylon - plans which would never be put into effect. No one knows for sure if he got sick or was poisoned; it could have been either. Regardless, his generals immediately began fighting over who should take...