During the Republic, Italia (which extended at the time from Rubicon to Calabria) was not a province, but rather the territory of the city of Rome, thus having a special status: for example, military commanders were not allowed to bring their armies within Italia, and Julius Caesar passing the Rubicon with his legions marked the start of the civil war.
The name Italia covered a portion of Italy that changed through time. At the beginning the name indicated the land between the strait of Messina and the line connecting the gulf of Salerno and gulf of Taranto; later Italia was extended to include the whole Italian peninsula, as well as the Istrian town of Pula; finally, Julius Caesar gave Roman citizenship to the people of the Gallia Transpadana� that part of Cisalpine Gaul that lay "beyond the Po"�, thus extending Italia up to the Alps.
With the end of the Social war (2nd century BC), Rome allowed the Italian allies to enter with full rights in the Roman society, giving the Roman citizenship to all the Italic peoples.
To cut a long story short, and much toing and throing (of Emperors as well as javelins!) In 476, with the death of Romulus Augustus and the return of the imperial ensigns to Constantinoples, the Western Roman Empire ends; for few years Italia stayed united under Odoacer rule, but later it was divided between several kingdoms, and would not be re-united for another thirteen centuries in 1871.