You're not going to get very far with this line of questioning, I'm afraid. Nationalism is almost universally thought to be a modern phenomenon, something that emerged out of the French Revolution, or some similar modern event. Nationalism is an ideology that believes the nation deserves self-determination expressed as political control of a national homeland. As the Declaration of the Rights of Man puts it:

The principle of all sovereignty resides essentially in the nation. No body nor individual may exercise any authority which does not proceed directly from the nation.

This is in contrast to sovereignty residing in, say, an absolute monarch, or a religious authority, or an entrenched aristocracy.

The nationalist argues that all members of a nation achieve the greatest political expression through participation in the life of the nation. They also (usually) argue that the nation is comprised of individuals who are culturally and perhaps linguistically homogeneous, and who have a shared historical memory not only with each other in the present, but with members of the nation in the past, and who have a shared historical or imaginary homeland, either in the past or the present.

This kind of ideology I don't think anyone anymore argues was present in the ancient world. There is, for instance, a proposition that the Senate and People of Rome are sovereign, but "People of Rome" refers to those who had citizenship. Rome was a multi-ethnic state, in myth, since Romulus opened the asylum. In fact, some theorists have argued that nations only come into existence because of nationalists, i.e. there is no primordial cultural/linguistic group people now belong to; that is an idea constructed by contemporary practice.

However, a lot of Classical scholars use the language of nations to describe Rome, and Anthony Smith, at least, has argued that nations are in fact ancient, and predate nationalism, and there are other models, like Kymlicka's "nation building state" that might be more useful than "nation" and "nationalism" for understanding what was going on in Rome. I've written a bit about these scholars before here and here.

I don't know what you mean by "positives" and "negatives" of nationalism. That's a contemporary debate for us, but I don't know if that's a sensible question to ask in an ancient context, even if nationalism was a thing.