President Obama proposed a sprint within a marathon Tuesday night. He will send 30,000 more U.S. troops to Afghanistan beginning next month, then begin withdrawing forces in July 2011. With that 18-month dash he intends to turn back the progress of the Taliban and stand up government forces to secure the country so that the Americans can begin to leave.

That is a very tall order indeed, given the challenges. In recent weeks we have urged the president not to send more troops to Afghanistan. We believe that the corrupt Karzai government is irredeemable, and without a legitimate regime that can call on the loyalty of the people, the American effort in Afghanistan has little chance of success no matter how many troops the president sends there.

What's more, the people of Afghanistan already doubt the commitment of the United States to the long-term struggle -- the marathon -- and to their welfare. Setting a date to begin withdrawal undermines the notion of long-term commitment, and tells the Taliban and al-Qaida how long they must wait out the U.S. escalation.

The president doesn't see it that way. He argues that the 18-month deadline tells Hamid Karzai that he must prepare quickly to shoulder the lion's share of the security burden. We concede there's some logic in that, though this particular deadline seems painfully short. It's not long to fortify an army and police forces in a nation where most people are illiterate, the notion of nationhood is a sometime thing, and where tribal loyalties trump all others.

However, the president knows that the American people are skeptical of prolonging this war and are tired of the sacrifices in blood and money. So he made sure to emphasize that Afghanistan will no longer be an open-ended American commitment, even as he rightly alluded to a long struggle against Islamist terrorism generally.

The president is undeniably correct that American security is at stake in Afghanistan and Pakistan because this is the epicenter of the violent extremism practiced by al-Qaida. We simply disagree that more American forces there can turn things around when, after eight years of warfare, the Afghan people have come to regard the Americans as infidel occupiers.

The president acknowledged the financial cost of this new surge, about $30 billion a year, though he didn't say how he would pay for it. Nor did he say how an American military already strained to the breaking point can sustain it.

The president has studied a difficult hand carefully. But we have doubts that this gamble will pay off.