Short answers, pulled from the story.
Italy held special political, religious, and financial privileges that no Roman province enjoyed. Roman magistrates exercised the imperium domi, a form of police authority, inside Italy, whereas in the provinces they held the imperium militiae, military power. This distinction was formally abolished when Diocletian reorganized Italy into the Dioecesis Italiciana in 293 AD.
Around 7 BC, Augustus divided Italy into eleven numbered regions, a system recorded by Pliny the Elder in his Naturalis Historia. The regions ran from Regio I covering Latium and Campania in the south to Regio XI covering Transpadana in the far north.
Augustus ordered three censuses; the surviving totals were 4,063,000 in 28 BC, 4,233,000 in 8 BC, and 4,937,000 in AD 14. Estimates for the total population of mainland Italy at the start of the 1st century range from 6,000,000, according to Karl Julius Beloch in 1886, to 14,000,000, according to Elio Lo Cascio in 2009.
In 286 AD, Diocletian relocated the western imperial residence to Mediolanum, now Milan, because the city was closer to the European frontiers where military pressure was concentrated. Rome's distance from the troubled borders had reduced its practical importance during the Crisis of the Third Century.
Alaric, king of the Visigoths, sacked Rome in 410 AD, the first sacking in nearly eight centuries. Rome was sacked again in 455 AD by the Vandals under their king Genseric.
The Western Roman Empire formally ended in 476 AD when Romulus Augustulus was deposed by Odoacer. Julius Nepos, recognized by Constantinople as the legitimate western emperor, survived until his assassination in 480 AD. Italy then passed to Odoacer's Kingdom of Italy and subsequently to the Ostrogothic Kingdom.