You'll have to explain that national interest bit some more to get me aboard. It's my belief that a drugs trade comes from having users, not suppliers. It's demand driven. If we fixed the Afghan drug supply side, we'd still have a problem. OK, it's preferable to have a stable country, anywhere, than what looks like an anarchic mess. But we are not applying that principle in every failing country, only one. One which has never really worked as a nation state in the traditional way, and whose disparate politics most of us will never understand. There is a lot more case to make on the national interest front. I'd be interested to hear it.
Politics is the art of the possible ... the argument on Iraq was similar ... why pick on that country when there are many other failed states? We went in because we could. Just because we cannot solve all the word's problems in one fell swoop does not mean that we should not tackle those which we can deal with.
In the event, Iraq has been a qualified success. Solving the problem in Afghanistan goes some way to resolving the problem in Pakistan ... on the basis that there is no solution to Pakistan without also dealing with Afghanistan. But look west and the other border with Afghanistan is Iran. With Iraq on one side and a stable Afghanistan on the other, that transforms the geopolitical situation in the Middle East - and makes intervention easier in the event - most likely - that the regime in Iran collapses.
To that extent, a solution in Afghanistan is also part of the broader Middle East solution, the resolution of which is most definitely in our interest.
That aside, as members of the "global community" we have broader responsibilities outside our narrow domestic interests. The foul rule of the Taleban causes genuine hardship and misery to an undeserving population and is a plague which, if unchecked, will spread. Are we to stand by and tolerate that when we have it in our power, as part of the wider community, to do something about it? Are we to define our interests and compassion solely by what happens inside this country?
As to the drugs issue, the same logic applies. We cannot solve all problems simultaneously. Fixing the drugs supply in Afg would be a good start. Usage is not only demand driven - it is also driven by price.
The plentiful supply drives down the price and makes the drug more accessible. Reduce the supply and the price goes up sharply, which will have an effect on the uptake of new users into the market. At £10 a "wrap", it is affordable to kids. Make it £50 a wrap and the commercial dynamics change significantly.