We are all familiar with Gibbon's Decline and Fall of the Roman Empire (1776). Though it is often the first book we think when discussing about the decline of Rome, it is by no means the first attempt in history that tries to explain what exactly happened to the Roman Empire in the Late Antiquity. Multiple Renaissance humanists like Leonardo Bruni and Poggio Bracciolini have pondered this question as well. However, the one who truly took it to another level was Flavio Biondo. As the title of his largest historiographical work, Historiarum ab inclinatione Romanorum imperii decades (The decades of histories from the decline of the Empire of the Romans, 1453), might reveal, Biondo built his whole work (or at least the first 10 books) around the subject. But why and when did Biondo think this inclinatio started? Below, I try to present how Biondo sees this topic.