The Westphalia Treaty which ended the 30 Years War was the basis of the concept of state sovereignty, when it became no longer acceptable for states to (openly) interfere in the internal affairs of other states. Setting up puppets, protectorates, and "spheres of influence" still occurred, but always holding the veneer of respect for the Westphalia Treaty. European powers were more than happy to openly ignore this in other parts of the world, until decolonization.
Your challenge is to prevent the emergence of any such concept to the present day, so that the world's major powers openly and publicly collect vassal states like stamps, and have no shame about installing puppet governments or negotiating spheres of influence.
Your challenge is to prevent the emergence of any such concept to the present day, so that the world's major powers openly and publicly collect vassal states like stamps, and have no shame about installing puppet governments or negotiating spheres of influence.