The key to a viable Afghanistan, one that is stable and peaceful and commands the allegiance of a majority of its citizens, is an honest and credible government. The Taliban and other insurgencies are a threat, but they gain traction only because Afghans feel that the government in Kabul does not look out for their interests. Corruption is a corrosive that destroys the legitimacy and viability of the government.

That is why the failure of Afghan President Hamid Karzai to crack down on corruption in his administration and among his political allies is so dangerous. Instead of cleaning up government, Mr. Karzai has been blocking efforts to clean out corrupt politicians. The result is not only eroding support for his government among Afghans, but deepening divisions among allied countries trying to stabilize Afghanistan.

The scale of the problem is staggering. Not only is there theft — it is reckoned that $1 billion intended to rebuild Afghanistan is illegally taken out of the country through Kabul airport — but the readiness of government officials to look the other way has led to an explosion in opium production, and a virtual flood of heroin around the world. The diversion and misuse of billions of dollars of reconstruction funds contributes to the strengthening of local warlords, a commensurate weakening of the power of the central government, and the alienation of growing numbers of Afghans. The Taliban may be harsh, but they are generally perceived as honest. (And sometimes corruption results in the diversion of money, weapons and information directly into the hands of the Taliban.)