Short answers, pulled from the story.
The conventional end of political control in the Western Roman Empire occurred in 476 when Odoacer removed Romulus Augustulus from his throne. The Senate sent imperial insignia to Constantinople, asking Emperor Zeno to accept sole rule over what remained.
Environmental shifts like the Late Antique Little Ice Age set in by 450 contributed to food shortages and population declines across Europe. Pandemics such as the Antonine Plague between 165 and 180 and the Plague of Cyprian from 250 onward further weakened imperial capacity.
Diocletian introduced a system called the Tetrarchy that divided power between two senior Augusti and two junior Caesars. This structure broke down within one generation, returning biological succession as the norm and leading to civil wars.
Large groups of Goths crossed the Danube River into the Eastern Empire in 376 while fleeing pressure from the Huns. Corrupt officials exploited these refugees instead of providing relief or resettlement, causing them to take up arms against the empire.
Constantine the Great declared official toleration of Christianity in 313, beginning a long process of religious transformation. By the end of the fourth century, Christianity had become essential for any ambitious civil official.