What act of Emperor Diocletian's reign significantly influenced the fall of the Roman Empire?

The main act that could be seen as the start to downfall of the Roman Empire was Diocletian's choice to divide the empire into 2 halves: East and West. Diocletian would rule and handle affairs over the East, while naming Maximian (aka Marcus Aurelius) to rule the West. Marcus Aurelius would become one of the most revered and respected emperors. Many say that he lived up to the idea of what a true "philosopher king" is and and should be. The idea of dividing the empire into smaller pieces was thought to be a best choice to better handle the barbarian invasions and more pressing local issues. The idea while good in theory was impractical. Both men would spend much of their reigns fighting invading groups. No peace would last for long.