The thing is, Italy was unified; it was the center of the Roman Empire for hundreds of years, and continued to be a Roman-style kingdom up to the second half of the tenth century. The political fragmentation in Italy following the tenth century is, in part, a consequence of it's own wealth, urbanization, and agricultural levels. These same factors strengthened it's lack of unity following the growth of trans-atlantic trade, leaving Mediterranean trade comparatively stagnant; as the Italian States stagnated, all they were able to do was maintain the status quo.

Let's begin at the beginning. The year is 476. The Western Roman Empire has lost Gaul (to the Franks) Iberia (to the Visigoths) and Africa (to the Vandals). Only Italy and Dalmatia continues to be ruled by Rome. Years of civil war and infighting has decimated the army, and a long succession of army-backed strongmen have taken turns ruling the empire. Many of these had mixed heritage: Romulus Augustulus, the last emperor, was himself a puppet of his father Orestes, the Magister Militum (head of the army) who had married into a Roman family but was not an ethnic Roman (and to be pedantic, the Eastern Emperor never even recognized Augustulus, which allowed Julius Nepos, his deposed predecessor, to set up a court-in-exile in Dalmatia from which he claimed to be Emperor of the West).

Flavius Odoacer, the de facto leader of the Foederati or ethnic "Barbarians" fighting for Rome (who made up most of the army), deposed Romulus Augustulus when por Romulus lost the army's favor. Odoacer then openly admitted that he was powerless beyond the alps and consequentially didn't call himself Emperor. Rather, he sent the imperial paraphernalia to Constantinople and asked to be recognized as "Consul", keeping all of the functions of the Roman Administration in Italy intact (understandable, Flavius Odoacer as was so romanized historians aren't sure which "Barbarian" ethnic group he belonged to). He initially played lip service to the pervious emperor Julius Nepos, minting coinage in his name, but may have been involved in the plot whereby he was murdered. Regardless, Odoacer eventually asked to be given the title of Patrician by the Eastern Emperor and be recognized as the ruler of Italy under the wing of a single Emperor in Costantinople.

So here's our first observation: while Spain France, and later England develop specific ethnic heartland (Frankish, Gothic, and Anglo-Saxon respectively) from which proto-nations can emerge, Italy never does depart from the vague concept of "Roman-Ness" which was a legal identity, more than an ethnic one.

So the Eastern Emperor Zeno had his own issues to deal with involving two Ostrogoths both confusingly named Theodoric (this in addition to social, religious, and political unrest in his own half of the empire). He takes action.

Although the Ostrogoths had been given lands in Pannonia in exchange for the defense of the upper Danube, Zeno came to the conclusion that they were more trouble than they were worth (fighting with him, fighting with each other, and in a general sense just being another annoying thing to have to deal with) so he convinced them to go conquer Italy. This way, they would stop annoying him, and he could also get rid of Odoacer, at this point another potential opponent.

Theodoric the Amal, King of the Ostrogoths, had been raised in Constantinople. His people were largely Romanized: they had lived within the Eastern Empire's borders for centuries. So when he defeated Odoacer and set up the first Kingdom of Italy, he did his utmost to further maintain the Roman institutions, law, and judicial system. However, he did not rule from Rome, rather, he ruled from Ravenna, seaside city conveniently defended by swampy surroundings, and easily supplied by ship.

Although Theodoric was de jure a vassal of the Eastern Roman Emperor, de facto the Kingdom of Italy operated entirely independently. (I suggest this book every time Theodoric comes up, he was awesome: Theoderic in Italy by J. Moorhead (Oxford University Press, Oxford, 1992).

This first Kingdom of Italy lasted to the sixth century. But starting from 535, the Eastern Emperor Justinian took advantage of infighting among the Romano-Gothic ruling class to re-integrate Italy into the Empire and sent an army to depose the King in 553 and control the peninsula.

However, the Byzantine Empire had a chronic manpower problem and lost Italy in less than a generation. Although Justinian was definitely a great wartime politician who was particularly good at picking the right man for the right job, his conquests stretched Byzantium's resources too far. Even sympathetic chronicles criticize him for being unable to defend the empire once he expanded it.

Thus, the sparsely defended Italian lands were easy prey for one of the last great migrations: that of the Lombards. The question, "where did the Lombards come from" is covered in Jorg Jarnut's History of the Lombards (Enaudi, Torino, 2002) but that's a story for another post. The Lombards would contest Italy with the Byzantines from the 6th to the 8th centuries. The Lombards moved into the Northern Italian Plains with relative ease, while the Byzantines would hold onto the Pentapolis, the city of Rome and its surroundings, and the easily-reachable Calabria and Puglia regions of Southern Italy.

Two centuries after the end of the Roman Empire and Italy was already deeply divided. The struggle would last two centuries more. The Lombards eventually "Went Native", imitating the Roman Imperial customs, establishing Mediolanum (modern Milan) as their capital, while still administrating Italy in various feudal subdivisions; indeed, had things gone differently, perhaps Italy would have survived as a medieval state, and centralized as the rest of Europe did over the course of the renaissance.

But any peninsula that can sustain two ethnic groups fighting over it isn't going to have a normal history.

The Kingdom of Italy would again be dismantled just when the Lombard King Desiderious (d. 786) got very close to finally kicking the Byzantines out of Italy. A successful offensive saw the encirclement of the Pentapolis (that to this day are called "Romagna" since the Byzantines were the last "Romans" to leave) which allowed Desiderious to march south against Rome unopposed. Fearing a loss of independence for the bishop of Rome, who had been pretty much guaranteed a free hand to rule the modern Lazio region in exchange for nominal Byzantine rule and military access, Pope Adrian cried "Uncle", excommunicated Desiderius, and asked Charlemagne to come to his aid.

Charlemagne, even though he was busy conquering what would later become Germany, was immediately flattered and marched for Italy.