In September 1864, Union General William Tecumseh Sherman ordered the civilians of Confederate Atlanta to evacuate so that his army could burn the city. Atlanta’s mayor pleaded for mercy; Sherman refused. “We must have peace, not only at Atlanta, but in all America,” Sherman explained. “To secure this, we must stop the war that now desolates our once happy and favored country.” In Sherman’s view, sparing Atlanta, with all its resources, might indeed be humane in the short run -- but would enable the rebels to prolong the war. And that was clearly the greater evil. Atlanta must burn. “War is cruelty,” the general memorably lamented, “and you cannot refine it.”

President Obama’s policy on Afghanistan -- up to and including last night’s beginning-of-the-end speech -- could use a dose of Sherman’s tragic wisdom. Sherman hated war, but there were things he hated more, such as failing to secure his country’s vital interests. And so he understood that the thing to do in a war is to define victory and fight like hell until you get it. You don’t ask your troops to battle a day longer than absolutely necessary – or a day less. You don’t split differences, or concoct exit strategies; you just accept the fight and the fact that “you cannot refine it.”

There was a time when President Obama seemed to understand this, too. “As President, I will make the fight against al Qaeda and the Taliban the top priority that it should be,” he said while still a candidate in July 2008. “This is a war that we have to win.” He meant to draw a contrast to the war of choice in Iraq, which he portrayed as a drain on resources that could have been better spent on Afghanistan.

In December 2009, when he announced a 30,000-troop “surge” to Afghanistan, Obama still emphatically insisted that “our security is at stake.” Yet this declaration was already tinged with ambivalence about the financial burden of the war, and included a proviso that the “surge” troops would start coming home in July 2011. The president was attempting to conduct war on a schedule, to fine-tune it – to refine it.

And now the pull-out. To be sure, there has been progress since December 2009. The Taliban has been knocked on its heels. Navy SEALs have taken out Osama bin Laden. But no serious observer of the conflict would say the U.S. has irreversibly achieved the goal that the president himself said was necessary to our national security: a stable Afghanistan free from the threat of the Taliban. The military leaders on the ground certainly don’t think we’re there yet, though they think we could be with a bit more time.

Nevertheless, we will declare victory and withdraw the surge troops, while leaving nearly 70,000 others to pursue – what? A final transition to full Afghan government control, and maybe a peace deal with all or some of the Taliban by 2014, when the administration’s schedule calls for everyone to go home.

I actually have a lot of sympathy for the president in this awful predicament, which he did indeed inherit, and which might not have been so awful if his predecessor had pursued a better Afghanistan policy. There are plenty of leaders in both parties who are tired of the whole business, and he can’t just ignore that. The president is right to worry about corruption in the Karzai government, to doubt that a wilderness like Afghanistan can ever truly be tamed, and to count the cost in lives and money.

But I have more sympathy for the 30,000 men and women who deployed, fought and suffered through a never-to-be-completed surge, and the 70,000 men and women who are supposed to stick it out during the pending Afghanization phase of this conflict.

This is what comes of constantly seeking a middle ground in war, a comfortable, calbrated path between going all out and getting all out. I thought we had already learned, in Vietnam and a hundred other places, that it’s wrong to prolong war on that basis. It is wrong to ask our soldiers – let alone those of our allies or the people of Afghanistan -- to risk their lives so that an uncertain government can hedge its bets. At least I’m pretty sure that’s what Sherman would have thought.

We’ll find out soon enough whether Obama has indeed retreated to a more defensible line – or simply granted a near-beaten enemy a new lease on life, of the kind Sherman refused the rebels at Atlanta. I hope for the former but fear the latter. When he was still alive, Osama bin Laden said that the U.S. bug-out from Somalia in 1994 proved that our soldiers were “paper tigers.” He bragged that “Muslims will be able to end the legend of the so-called superpower that is America,” just as the Afghan mujahideen had brought down the Soviet Union. Bin Laden is dead now, but many around the world will read Obama’s speech last night as posthumous vindication for the terrorist. That’s dangerous.

Certainly those who wish us ill in Afghanistan, Pakistan and elsewhere were listening closely as the commander-in-chief announced that the U.S. must henceforth focus on nation-building at home, stimulating our economy through deficit spending on clean energy rather than deficit spending on defense.

In Sherman’s time, many men lucky enough to return home were missing arms and legs when they did. Miserable as they might have been, at least they never had to guess why their commander and their country had asked them to pay that price.

And today? A little while ago I met a young soldier, back from service in the Afghanistan surge. He hobbled on his prosthetic leg into a frozen yogurt shop where my kids and I were enjoying life. We see a lot of guys like this in my neighborhood, which is not far from a big, busy military hospital. I bought him a sundae, exchanged pleasantries -- and wondered to myself just what purpose his sacrifice had served.

After listening to the president, I’m still wondering.