Not a topic I dabble in frequently, it being a royal prerogative and also something I know relatively little about...
But doesn't the current crisis in Georgia indicate how impossible it is for us to run an ethical foreign policy? If a country that didn't have an enormous army and a substantial nuclear arsenal were invading and bullying its neighbours in the way that Russia has been over the last week or so, there is no doubt in my mind that even with our overstretched military and shortfall in resources we would have gone to war. I feel quite strongly that we ought to have done.
But we can't, because we can't fight Russia.
Foreign policy highlights the enormous gaps that exist between ideals and reality in our fallen world. There are two main gaps. I guess the ideal, for pretty much anyone, would be pacifism. It would be lovely if there could just be peace. But we have to accept that in a fallen world, there will be wars (and, indeed, rumours of wars), and to refuse to stand against aggressors is, I think, culpable. So, there's the gap: in principle, everyone is against war, but in practice sometimes we have to fight. Just war theory comes from this recognition, and becomes in fact a new 'principle'. Our high-level principle is still true, but it is unworkable in a fallen world. So, the new principle is that we engage in defensive and just wars.
And then there is a second gap between this new, just war principle and the practice on the ground. This obviously occurs when we engage in unjust wars, but it can also occur when we refrain from engaging in just wars due to lack of ability or will.
Of course, we can't surmount the first 'gap' - all we can do is say 'come, Lord Jesus!' - but can we with any more hope try to overcome the second gap, and run a really ethical foreign policy? I doubt it.