In Mazzini's time, people thought that if every people (nation) had its own state, wars would end and everyone would cooperate. In reality, patriotism evolved into a more or less exacerbated nationalism at the beginning of the 20th century, and this became the fundamental cause of history's greatest tragedies: the two World Wars.
The fact is that the principle of nationality is extremely vague. It is meant to indicate a common cultural community (a common mentality), but in reality, different mentalities can be found within every nation, sometimes leading to bloody clashes. At the time of the Unification of Italy, for instance, a Lombard was culturally more similar to an Austrian than to a Calabrian, and so on.