Of course intervention in all cases and all places will never be possible or desirable. There are the realities of power. Back in the 19th century the Royal Navy was much tougher on Portuguese slave traders than on French ones. Similarly, there will be no intervention on behalf of Chechens or Tibetans. And, in probably the most important lesson from our recent spate of interventions, some societies, and problems, are simply too complex and messy for the international community to be able to make a difference. The "nation building" which we have learnt will be needed after any substantial intervention (and is often more demanding than the original intervention itself) can simply prove beyond our powers and patience. Some evils may need to be tolerated because the alternative could be worse. (Which is preferable in Egypt: the military or Islamism?) Total moral consistency is a mirage. 

But that is not an argument for doing nothing at all. As the Bosnia, Kosovo and Sierra Leone examples underline, there are places where external intervention can make a real and beneficial difference. Such cases will not be frequent, but when they appear both pragmatism and conscience argue for action. Indeed the sheer readiness to intervene can inhibit global evils. The West's early failure to stop Assad can only have encouraged other dictators to follow in his bloody footsteps.

Inescapably, it is the United States, and its close allies such as the UK, that will have to carry the burden. We are the only countries with both the global reach and the commitment to international standards to be able to act both rightly and effectively. The role of global policeman may be an ungrateful and expensive one, but a world where someone takes that role will undoubtedly be safer and more secure (including for us) than a world where no one does. With rising Russian and Chinese obstructiveness in the Security Council, that is precisely the alternative that we face. 

The core actor in all this is, of course, the United States. But we should not underestimate the UK's influence. I worked in the British embassy in Washington 2001-04, and saw, particularly post 9/11, how close and wide-ranging our relations are, and how congruent our objectives. It was largely British arguments that drew the Americans into the Kosovo and Libya exercises. Given their obvious ability to go it alone, the Americans attach remarkably high political importance to having allies alongside them; and what the British do is noticed. If we want them to continue carrying the burden of global order, then we need to be there too.