Ever notice how, apart from international law, countries of the world have different sets of law but can compete against each other like companies would in a free market? Is this because each country, for the most part, respects the sovereignty of its competitor state, just like companies respect the endeavors of other companies?
I think countries get along because they can come to mutual strategies to tax their populations. When these strategies are unavailing then cooperation becomes difficult. As such this isn’t that analogous to what happens in a market -state to state competition only enriches the state but not the people and it leads to more disorder than what a pure free market would have.
Not only do countries respect the sovereignty of other states, their sovereignty comes from other states. It is only by mutually supporting each other that they can preserve their collective monopolies.