Law as effective norm versus law as ideal

Further to my post on Raphael Lemkin and the state of international law regarding genocide, Andrew Sullivan makes an interesting remark in passing in his column in the Sunday Times today. Andrew is writing about President Obama's recent appointments of Samantha Power and Susan Rice as, respectively, US ambassador to the United Nations and US national security adviser and he explains the move as designed 'to ensure that the Democratic party's liberal interventionists stay in the Obama tent as the world simmers and occasionally boils'. What caught my eye, though, was this (£):

[B]oth women are strikingly out of line with the president's cautious realism in dealing with intervention abroad. Power wrote a book about genocide that simply assumed it was the West's job to intervene anywhere to prevent it. Rice still smarts from her time in the Clinton administration when the Rwandan genocide took place. She later remarked: "I swore to myself that if I ever faced such a crisis again, I would come down on the side of dramatic action, going down in flames if that was required."

I'm interested in the phrase 'simply assumed it was the West's job to intervene anywhere to prevent it'; because one could argue that, in view of the UN Genocide Convention, that's a reasonable assumption to make, and not just about the West. Article 1 says: 'The Contracting Parties confirm that genocide, whether committed in time of peace or in time of war, is a crime under international law which they undertake to prevent and to punish.' So, if this article of international law were already an authoritative and effective legal norm, the assumption in question - of Samantha Power's book - would not be a wayward one at all. Unfortunately, as things stand, the UN Genocide Convention is more like an ideal - something the international community is committed to as a future aim, but one that for the present is frequently blocked by considerations of great power politics. In this light, simply assuming it is the West's job to prevent genocide becomes more problematic. Yet the world is a lot worse for this fact than it would otherwise be.