When he ordered his escalation of the war in Afghanistan, President Barack Obama pledged that U.S. troops “will begin to come home” in the summer of 2011. Discouraging reports from the war zone should make him more determined to keep his promise — and Americans more insistent on holding him to it.

In his Capitol Hill testimony this week, Gen. David Petraeus — the godfather of Obama’s 30,000-troop Afghanistan surge — sought mightily to carve out some wiggle room. “We have to be very careful with timelines,” he told the Senate Armed Services Committee. The July 2011 deadline for beginning a troop withdrawal depends on the assumption that “conditions” are favorable, Petraeus said.

But wait a minute. Another way to describe a withdrawal deadline that is based not on the calendar but on an amorphous and elusive set of “conditions” would be to call it an open-ended commitment. This is precisely what Obama said he was not giving to Afghanistan’s corrupt, feckless and increasingly unreliable government.

There were basically two reasons for establishing a firm timeline in the first place. One was to mollify skeptical U.S. public opinion, which had begun to associate the war in Afghanistan with such concepts as “quagmire” and “Vietnam.” The other was to apply maximum pressure on Hamid Karzai, the mercurial president, to shape up and get with the program.

Which he has not done. Karzai, who seems not to have gotten the memo on how a U.S. puppet should behave, alternates between grudging cooperation and petulant defiance. Most alarming is that Karzai is effectively sabotaging the effort to win hearts and minds in Kandahar, the heartland of the Taliban insurgency, by leaving the local power structure in the hands of his thuggish and corrupt half-brother, Ahmed Wali Karzai.

In Washington, the hawkish interpretation of events is that the timeline itself is now the problem — that, in the words of Sen. John McCain, it tells “the key actors inside and outside of Afghanistan that the United States is more interested in leaving than succeeding in this conflict.”

This sounds like a reasonable argument until you think about it. Karzai, the Taliban, the warlords and the Afghan public already know that the U.S. and NATO forces will leave someday. The only way to convince them otherwise would be to announce that we intend to stay forever — and clearly that’s not the case. From the Afghan point of view, it doesn’t make much difference whether the interlopers depart in one year or in five.

It might make a difference, of course, if there were an honest, capable Afghan government that could use more time to build its capacity and earn the people’s trust. Everyone knows, however, that such a government does not exist.

McCain complains that all the competing Afghan factions are “making the necessary accommodations for a post-American Afghanistan.” But this outcome is not only inevitable, it’s what we claim to want. Sooner or later, there will be a “post-American Afghanistan,” and some measure of power and influence will be held by Afghans who now consider themselves loyal to the Taliban. Corruption will not vanish, nor will the poppy and marijuana fields, nor the system of clan-based loyalties that has survived a millennium’s worth of foreign invasions.

It’s not that Afghanistan is some sort of hopeless case. It’s just that thinking that a U.S.-led experiment in nation-building — and that’s what we’re attempting, even if we call it counterinsurgency — can impose a whole new organizational template on the place in a year or two, or even 10, is pure fantasy.

Whether or not Obama adheres to his announced deadline matters less to the Afghans than it does to us. U.S. casualties are increasing, as was anticipated; Obama has tripled U.S. troop levels since he took office, and the battle for Kandahar will be bloody. Our European allies are squirming, balking, complaining and looking for the exit. As time goes on, this will become even more of an exclusively American war.

The question is how much more the war will cost in precious young lives and in scarce resources. Obama won the nation’s forbearance by making a promise that the inevitable withdrawal of U.S. troops would begin next year. Americans should expect him to keep his word — and insist that he does.

Eugene Robinson’s e-mail address is eugenerobinson(at)washpost.com.