That has meant Mr. Obama had to wait well past the flawed Aug. 20 elections and then later Mr. Karzai's admission of massive fraud in the ballot count. Only now, just before his inauguration Thursday to a second five-year term, did Karzai finally make fresh promises to rein in graft. One pledge includes new anticorruption units among police, prosecutors, and judges.
But then, after the swearing-in, Secretary of State Hillary Rodham Clinton asked him to make these units "credible" with "follow through." And the US ambassador to Kabul, Karl Eikenberry, said "deeds are required."
One deed in high demand is the appointment of "clean" cabinet officials and others by Karzai, down to the provincial level. That would necessarily include the sidelining of corrupt warlords that Karzai brought into his political tent in order to get reelected.
Obama makes an implied threat of withholding US troops and aid to Afghanistan if Karzai doesn't produce a government that is seen as moving strongly against corruption, or as Ms. Clinton put it, a "new compact with the people of Afghanistan."
Growing resentment against government corruption among Afghans runs the risk that they will turn to the Taliban for basic governance or, just as bad – or worse – turn against foreign troops, either violently or simply by not cooperating in the war on the Taliban.
At the same time, Obama's implied threat runs another risk: If Karzai doesn't produce results, will the United States withdraw from Afghanistan?
By most estimates, the Taliban would win over much territory from the still-weak, secular Afghan forces, and it would possibly once again provide a haven for Al Qaeda to train global terrorists.
Obama says he will make his decision on additional troops in a matter of weeks. And he might spell out his benchmarks for progress against corruption in making that announcement.
But before he does that, he must see the measure of Karzai's commitment and courage in this struggle, and whether he cuts himself off from notoriously corrupt leaders.
The US is not blameless in this tense time of decision. Floods of foreign aid to private groups and the Afghan government could have been better monitored since the 2001 US-led invasion to prevent money from going into the pockets of local officials. And Western consumption of heroin has only increased the opium trade in Afghanistan and heightened official corruption. The US must do more in both those areas.
Patience is needed to restore a society ripped by three decades of conflict, one that has lost much of the traditional values that once helped keep dishonesty in check.
Obama must be clear on what he expects of the Karzai government, and support Karzai as long as he is making progress against corruption. No government is totally clean, but as long as Afghans see hope of better governance, they won't give up. And the US shouldn't either.