in the early medieval period, northern italy was a part of the feudal system like most of europe say around charlemagne's time and was part of what became the HRE. by the time of the late medieval period, northern italy was a collection of independent city states. there were no other region of western europe, except for the northern italian's neighbors, the swiss, who managed to break away as completely as they did from one of the major feudal states. there were free city states in other parts of the continent, as for example in germany, but they weren't heavily concentrated in one region like they were in northern italy.

so what were the causes that enabled northern italy to break away and stay independent from powerful neighbors like the HRE, the byzantines, the french and the papacy?

1. was it economic? if so, there were other rich cities regions (flanders, holland) that were usually under the control of some major power.

2. was it military? did their expertise with crossbows make them too dangerous for knights, the way the pikes helped the swiss?

3. was it religious? the papacy needed some weak buffer zone between them and the HRE and fought to keep it so?

or something else entirely?