This analogy breaks down because the UN is not a police force: it is a structurally flawed club reflecting geopolitical realities from 50 years ago. For example, democratic nuclear-armed France (population 60 million) has a permanent seat on the Security Council while democratic nuclear-armed India (population one billion) does not. Due to its peculiar veto system it is demonstrably unable to respond effectively to crises. In a month when we remember the tragedy in Rwanda - and watch similar events unfolding now around the world - it is no suprise that reflective people the world over are questioning the UN's axiom that national sovereignty trumps human rights.
Whether or not we support direct military action without UN approval (such as in Kosovo), we should better think of the UN as a toothless Neighbourhood Watch committee operating in a police no-go zone. In this Hobbesian neighbourhood, sadly, posses are the only way to stop abuses within households. Before we take the next step of forming a global police force we must reform the UN to make it representative, resourced and responsible.