I think it is difficult to contest that the most important state player in world affairs over the last one hundred years - and consistently so over this period - has been the United States of America. World War I - into which, to borrow from Christopher Clark's justly celebrated book, we "sleepwalked" - marks a useful starting point. It is not only the fairly important role America played in bringing WWI to an end that signals the beginning of this era, but also the no less important role it played in shaping the aftermath. Wilson's 14 points were considered at the time "idealistic" by some of the yet-to-be "Old Powers". But by dismantling the Ottoman Empire through the principle of self-determination (not at that time a universal legally binding norm) it was an early swallow to the demise, a mere generation later, of all other colonial empires and the truly decisive reshaping of the balance of power in the post-WWII world. The US played an equally cardinal role in ideating and realizing the United Nations Organization and the Universal Declaration of Human Rights - two lynchpins of our current world order.
That opening gambit to the American century is emblematic, in my view, of the entire Pax Americana epoch: American action in the international sphere has always had a strong dose of idealism (to be sure sometimes misguided) mixed in with the normal national self-interest which is the usual stuff of international relations, remembering that if we disaggregate the state, as we almost always should, what passes as "national interest" is often but "special interest" of certain sections in society.
I know that the various schools of "realism" tend to pooh-pooh any deviation from interest analysis. Generally speaking, I find the emphasis on interest/power as an explanatory device to