The security establishments of Russia and the United States identify the threat of the spread of weapons of mass destruction as sufficiently serious to compel cooperation despite differences on other matters. Once there is the political support for cooperation the diplomats can exercise their skills successfully.
It's no surprise that the powerful both set the rules and break the rules with impunity. The world system isn't presided over by Miss Manners. For small countries like Greece, there's not much room for maneuver between the regulations of the EU and the general parameters established by globalization. There isn't much room for democracy either, as Greek citizens discovered when they voted in Syriza and attempted to vote out austerity in the more recent referendum. Iran, a larger country that plays a strategic role in the Middle East, has considerably more room for maneuver than does Greece. But it too cannot unilaterally remake the rules of the game. It can only negotiate the best deal it can. In the end, it must open itself up to the kind of inspection regime that more powerful countries would never tolerate.