While watching the utterly gripping movie “Lone Survivor” recently, I comforted myself by noting that the four Navy SEALs engaged in a desperate firefight with the Afghan Taliban were all volunteers. They asked for this, I told myself. They were not draftees yanked out of civilian life and compelled to fight a war they could neither understand nor win. They had asked for this, I insisted, but I knew all the time that this was a lie. They had volunteered, but certainly not to die and certainly for no purpose.

Okay, I know this is only a movie. But it is faithful to the book of the same name , which is faithful to the 2005 mission called Operation Red Wing that was intended to take out a Taliban commander. The title “Lone Survivor” pretty much says what happened, but you owe it to the SEALs and to their families to see the movie. The ending is not in doubt, but the reason for their sacrifice undoubtedly is. Afghanistan is a war searching for a reason.

All through the movie, I kept asking myself, Why? What are these men fighting for? Once, I knew the answer. After Sept. 11, 2001, I wanted to wipe out al-Qaeda and kill its Afghan hosts, the Taliban. Even before the terrorist attack, reports of the Taliban’s treatment of women — stonings, public executions in the soccer stadium, etc. — and beheadings of men convinced me that it simply had it coming: Send in the Marines.

But U.S. fighting units have been there since 2001. The initial mission — the destruction of al-Qaeda in Afghanistan — was completed long ago. The Taliban and its allies remain, but unlike al-Qaeda they are indigenous and, seemingly, undeterred. They apparently have an unlimited supply of suicide bombers (who are these people?), and they continue to inflict mayhem on Afghans and foreigners alike. This month, the Taliban struck a Kabul restaurant that draws a Western clientele and killed at least 21 people. The attack by gunmen was preceded by a suicide bombing.

Bob Gates, in his memoir “Duty,” depicted Barack Obama as a commander in chief whose policy in Afghanistan was to do as little as possible — simultaneously ordering a surge and announcing a pull-out date. Gates, then defense secretary , was appalled: “The president doesn’t . . . believe in his own strategy and doesn’t consider the war to be his.”

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Well, the war is not Obama’s. It is George W. Bush’s — one he interrupted to mindlessly chase after Saddam Hussein. But Obama embraced the Afghanistan mission and then, apparently, never knew what to do with it. I don’t blame him. Afghanistan is an arid Vietnam, a quagmire presided over by the petulant and unpredictable Hamid Karzai. For Obama, Gates wrote, “it’s all about getting out.”

The quote is pithy, but the observation is banal. It was clear back in 2009, when Obama ordered his surge in Afghanistan, that he had no stomach for continuing the war. The war goes on and on and has now become fused with the futility of Iraq — 2,307 Americans dead in Afghanistan, 4,489 dead in Iraq, an incomprehensible waste of lives.

The administration wants U.S. troops to remain in Afghanistan (the Pentagon has suggested 10,000). It has its reasons. The country was once a terrorist base and could revert. The necessary pursuit of the remaining Sept. 11 terrorists is best based in the region — as are U.S. drones — and without an American spine, the Afghan army could collapse. That would permit the return to power of the Taliban and the abandonment of women and girls to frenzied misogynists. That, though, has nothing to do with realpolitik, just real life. Soon, the music will die and we will have to avert our eyes.

But as Gates insisted, Obama has failed to make these or other arguments. “He needed to say publicly why the troops’ sacrifices were necessary,” Gates said of the president. Gates made that point several times, and he is right. Maybe, though, Obama is cautioned by the experience of Lyndon Johnson. On July 28, 1965, LBJ began a news conference by addressing the question of “why we are in Vietnam.” He never supplied a satisfactory answer.

In the movie theater, I watched two films at once — “Lone Survivor” on the screen and Vietnam in my head. On the screen, as in reality, men fought and died — and, as with Vietnam, I no longer knew why. One man survived the battle. The rest were lost — as is the reason for the war itself.

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