Here are lines of thought:
If peace appears as a choice, then opportunities where it will not apply will always present themselves. In any case, even believing that peace is the answer will lead people and countries to war and failure of communication in the name of an ill conceived peace. For example, if my thoughts get rowdy, sometimes in the name of peace I can tell my mind to shut up, but this does not promote any real peace.
Other things that come to mind is that the individual will always perceive the need to preserve itself even in the absence of threats, simply because that is implicit in an individuality claim, so a minimal self-defense force would seem likely for countries, even where countries all get along. It is simply implicit in an individuality claim. To not consider self-defense appears to be based on a nostalgic desire to return to innocence, but that "innocence" would largely negate individualities, and it should not be forgotten that this amounts to the negation of consciousness and thus death. So it is a kind of mirage to chase this innocence. It is very viperous.
If the question considers the number of countries, it is not relevant. If the United States were the only nation, the states would still have differences like countries. If Colorado were the only inhabited area of the planet, there would still be city-states, and each would be inclined to their own militia. Each would, just as now, have their own political individuality. If Los Angeles were the only inhabited area of the planet, there would still be the politics of the neighborhoods. If a neighborhood, the streets. If a church, the congregations. If a company, the departments. If a family, the individuals. And so on. Division is implicit ... and it is good. There is no pessimism here.