Well, I’ve thought a great deal about how to effectively link empire to a broad social contract theory and, unfortunately, I haven’t come up with too much that’s of any real value. I think part of this is because social contract theory tends to be based on the idea of some level of equality between ruler and ruled, and empires in themselves are an aberrational product where power is concentrated in the hands of nation(s) at the expense of others. Fundamentally, the basis of the relationship is about inequality.

Certainly, I think in many ways the United Nations and its antecedents represents at least a cursory effort to—if not bind great powers to social contracts—provide a mechanism to reduce international tensions, and as a consequence has the effect of legally recognizing the sovereignty of all countries being equal. Realistically, we all know that sovereignty is not equal, and that is reflected in the fact that there are five permanent members in the UN security council.

Probably, if there has been any significant development that has altered the relationship in empires it’s probably been the development of nuclear weapons. This means that nations that would otherwise be gobbled up as a backward province in the empire’s hinterland instead become major international players who have the ability to, in the event of direct annexation, possibly take down the rest of the world with them. Even though this can be seen as a solution to empires that absorb other nations territorially—in other words, through direct military conquest—for reasons that I hope are obvious I hope this isn’t a solution that is readily adopted.

However, you’re really querying whether there is something that can be done to fundamentally change the relationship between polities irrespective of changes of economic strength, military power, political developments, etc. I think to really answer this question you have to sit back and ask think about how nations are internally run. For example, if one follows the believe that nations are in essence, amoral, Machiavellian entities governed by characters who by-and-large favor the same traits, then ultimately there might never be a behavior change that could bring about the social contract change you seem to insinuate. Instead, the answer must be risk-reward—prohibitively costly military costs, stronger and more authoritative international organizations, equivalent economic leverage among weaker nations, etc. If one believes, however, that nations are run by people who apply their own thoughts, beliefs, ideas, and ethical systems to political questions, then I think the question becomes more about how to foster a normative appreciation for the value of justice between nations to such a degree that such values take precedence over—possibly—political gain at the ballot box. What’s the solution there? Education? An emphasis on some form of ethical political philosophy? I really don’t know, and the more I look at it, honestly, the less confident I am in those solutions. I think this is especially the case because I think democratic empires tend to have a great deal more incentive to exploit less powerful countries because, when all is said and done, they are unrepresented in elections.

Thus, I think, in a real world where nations are run by human beings who do have their own thoughts and feelings, but also look at issues pragmatically, you need some combination of the above solutions—however ineffectual they might seem. Unfortunately, many countries hoping for leverage against empires will not hold their breath for such a profound change in the moral political culture of a state. Instead, for these countries, the most effective leverage against an empire will still be weapons of mass destruction and limited resources (oil). I hope that, at least, explores your question.