Last December, when he announced his decision on Afghanistan, President Obama solved one problem and created another. It was quite a feat, all the more so because he did it in a single paragraph:

"As commander-in-chief, I have determined that it is in our vital national interest to send an additional 30,000 US troops to Afghanistan," Obama said at West Point. "After 18 months, our troops will begin to come home."

The first sentence was the surge; the second undercut it by announcing an exit date. The two ideas, aimed at satisfying very different audiences, were always in conflict.

Finally, the conflict must be resolved.

As we approach the halfway point of those 18 months, it is no longer possible to finesse the question of whether Obama intends to succeed in the war, or merely to fight it for a specified period of time. It is either a vital national interest or it's not.

Our troops need to know, and so do our allies and enemies. We the people also deserve a clear answer.

That is the fallout of Obama's decision to replace Gen. Stanley McChrystal with Gen. David Petraeus. With our casualties rising to record levels, a new commander cannot be expected to take the field with the exit date hanging over the mission.

There is too much to do, and too little time to do it, if the date is firm. If it's not firm, Obama should say so.

For his part, Petraeus, the commander who saved our mission in Iraq from defeat, indicated in Senate testimony yesterday that he supports the timetable, but sees it as flexible and based on circumstances. Obama has only suggested as much.

After blasting what he called "obsession" over the date, Obama said last week:

"We did not say that starting July 2011, suddenly, there would be no troops from the United States or allied countries in Afghanistan. We didn't say we'd be switching off the lights and closing the door behind us."

No, but he came damn close, and Vice President Joe Biden came even closer, telling a reporter to "bet on it" that many troops would be leaving then. This is not a matter of semantics. The expectation of retreat has taken on a life of its own and is changing the dynamics of the war.

The recent flirtation of Afghan President Hamid Karzai with elements of the Taliban, and the push of some Pakistani officials for the Taliban to join a power-sharing arrangement, both reflect calculations of a post-American Afghanistan.

The irony is that, in general, Obama has been very aggressive in taking the fight to the enemy. The surge and the extensive use of drones to go after Al Qaeda sanctuaries in Pakistan and Afghanistan are making America safer from terror attacks at home, according to government and law-enforcement officials.

Yet the cost in American soldiers and treasure is far too high if there is no intention of securing lasting gains in the war zone. Obama, of course, created the conflict because he aimed to satisfy both hawks and doves. And he did -- up to a point.

The point has passed. Whatever the exit date gained him temporarily with the anti-war left that was vital to his election, the expectation is now an impediment to success. He needs to choose.

If he were so inclined, there is a model for what Obama should say. Asked about the endgame in Iraq, President George W. Bush would often say America "will stand down when the Iraqis stand up."

That is essentially our policy in Afghanistan, and we now have the best possible commander to achieve it. All that's missing is a president willing to say so.

Michael Goodwin is a New York Post columnist and Fox News contributor.

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