Barack Obama has now taken responsibility for the war in Afganistan, just as Richard Nixon inherited Vietnam from Lyndon Johnson 40 years ago.

Nixon widened the conflict into neighboring countries, and tried to turn the war over to the South Vietnamese. Obama says he would go into Pakistan in hot pursuit of Al Qaeda, and he is sending 17,000 more Americans into Afghanistan while hoping to build up the Afghan army. Both the Afghans and the South Vietnamese were handicapped because the enemy enjoyed safe sanctuaries in Pakistan and North Vietnam, respectively.

Obama campaigned on the proposition that Iraq was a mistake and a diversion from our true goal, winning the war in Afghanistan and defeating Al Qaeda.

I believe Obama was right in the sense that the Bush administration took its eye off the ball when it invaded Iraq. Iraq had no relevance to the terrorist attacks of Sept. 11, 2001. But one worries that Obama’s call to win the war in Afghanistan — at least in the way winning has been defined — may be unachievable, and may even contort and bedevil his presidency the way Vietnam did to President Nixon.

Under George W. Bush, the goal in Afghanistan seems to have been to create a centralized government and a Western-style democracy, with full human rights, in an environment in which Al Qaeda cannot operate. This is beyond America’s power.

This week the Obama administration is beginning a review of the Afghan-Pakistan conflict, and will be looking for ways to lower the goal posts by setting more realistic ambitions. It may have to concentrate solely on keeping the Taliban from overrunning both countries and denying Al Qaeda tranquility and room to maneuver.

Obama should resist the urge to follow the Bush administration in trying to restructure an old and tribal society according to Western norms. What he should be doing is taking Defense Secretary Robert Gates and the joint chiefs aside and saying: “Give me your thoughts and best ideas on when and how we can responsibly withdraw troops from Afghanistan and how many years will it take.” For the truth of the matter is that we have been there too long.

What needed to be done in 2001 needed to be done quickly, without staying on and reinforcing the impression of an occupation. Afghan President Hamid Karzai is right to worry that more U.S. troops could further alienate Afghans. Yet Americans have the grim memory of how their disinterest in Afghanistan after the Soviets withdrew created the conditions for a Taliban-Al Qaeda takeover. But the Afghanistan operation now involves 40 countries, and Karzai is also right to ask: “So many messengers. What is the message?”

When contemplating Pakistan and Afghanistan, I often turn to the writings of John Masters, who served in the border wars of the British Raj 80 years ago. Then, as now, the Pashtuns — they were called Pathans then — were the problem in the lawless frontier between Pakistan and Afghanistan. Today they form the base of the Taliban insurgency.

Britain could have attempted to “disarm the tribesmen … and introduce full-scale administration as it was known throughout the rest of India — the law, the lawyers, the taxes, the police and the rest, all alien to Pathan (Pashtun) tradition and spirit,” Masters wrote. Instead the British thought it wiser to attempt to pay off the tribes, and keep a force on the frontier in case the tribes didn’t stay bought.

“The government also tried to remove the conditions that made the Pathan such an awkward element … Its efforts never met with much success,” Masters wrote. “Consent is part of a democracy, and it was neither easy, nor, perhaps, right to force the Pathans to attend school, give up vendettas, and become peaceful farmers, when the old bloodthirsty ways constituted for them life, liberty, and the pursuit of happiness.” Trouble is that nowadays the old bloodthirsty ways have consequences far beyond the frontier.

The latest American and Pakistani idea is to form and pay local militias to be loyal to the government, as was done with success among the Sunnis in Iraq. But there are worries that armed tribesmen might end up abusing the local population.

The British tried arming Pashtun tribesmen into a defense force, but the militiamen were afraid of starting blood feuds and reluctant to fight other Pashtuns, according to Masters. And they “always seemed to be permanently in a temper about pay and promotion. A high proportion of the stray shots fired at the (British) army were fired” by these defense forces.

“We always thought they were an unmitigated nuisance, but probably a necessary step in local responsibility for law and order,” Masters wrote. Let’s hope the American and Pakistani efforts work better than they did under the British.