Ambassadors are supposed to be diplomatic. But, by all accounts, there was nothing particularly sensitive about the opinions outlined in the leaked memo from the American Ambassador to Afghanistan, Karl Eikenberry. In the note to the White House, Mr Eikenberry is said to have articulated grave concerns about Hamid Karzai's government and argued against sending more US troops to the country.

As a former military commander in Afghanistan, Mr Eikenberry's opinion will have particular weight in Washington. And this dramatic intervention clearly represents another serious challenge to the case, made by the US commander General Stanley McChrystal, for a further 40,000 US troops to be dispatched to Afghanistan.

Yet a danger lies in an exclusive focus on the issue of troop...