No matter how many troops President Obama orders to Afghanistan, victory will also require a surge across the Pakistan border that the Taliban and al Qaeda—but not American GIs—cross easily. The President knows this, but he hasn’t made Pakistan’s help any easier to obtain by signalling his intention to draw down a mere year after his surge troops arrive in Afghanistan.

Pakistan has slowly expanded its cooperation this year as its public and military have awakened to the threat from their own Islamist militants after a spate of terrorist attacks, including on the military headquarters in Rawalpindi. Long portrayed as noble bearded mountain fighters in Pakistan’s press, the Islamists are at last seen as an existential threat to Islamabad and Lahore. And this year the military has pushed the Pakistani Taliban from the Swat Valley and South Waziristan and, in contrast with past offensives, hasn’t for now ceded back the ground in a misconceived truce. This is progress.

But so far the generals have refused to take on other Islamists they don’t view as a danger and have long cultivated as strategic assets—that is, the Afghan Taliban. This means the Taliban government in exile in Quetta, the capital of Baluchistan province, and Afghan insurgents loyal to the ailing Jalaluddin Haqqani and his son Siraj based in North Waziristan. The so-called Quetta shura is led by deposed Taliban leader and Osama bin Laden ally, Mullah Omar, who fled in 2001 and now directs the fighting in southern Afghanistan from Quetta. The Haqqani network is the largest insurgent group in eastern Afghanistan.

We’re told that Pakistani President Asif Ali Zardari, in a private letter to Mr. Obama earlier this month, promised to take the fight to North Waziristan and Baluchistan. Merely to have a Pakistani politician acknowledge the existence of the Quetta shura counts as progress. The Pakistanis are reluctant to arrest their longtime proxies, Haqqani or Omar, but they could at least disrupt their headquarters and make it harder to operate from Pakistan.

As ever, the final decision rests with the Pakistan military led by General Ashfaq Kayani. According to a story in the New York Times, he has resisted the entreaties and told the U.S. that his troops have their hands too full with their own Pakistani Taliban to expand their operations.

The head of the U.S. Central Command, General David Petraeus, and the Chairman of the Joint Chiefs Mike Mullen visited Pakistan last week to nudge some more. Perhaps they used the opportunity to express U.S. frustration about official Pakistani complicity in the deaths of American troops in Afghanistan. Such messages need to be sent, though the best way is in private.

If Pakistan truly has given up on its old double game of claiming to back America while allowing a Taliban sanctuary within its borders, now would be a good time to show it’s serious. If not, the U.S. has leverage with Islamabad through foreign aid, as well as various military options. U.S. drone strikes can be expanded, including for the first time to Baluchistan, and special forces might be deployed across the porous border.

Both carry diplomatic risks. Though drone strikes have killed about two dozen civilians according to one Pakistani government estimate, the country’s press loves to exaggerate the toll to embarrass the government and stoke anti-Americanism. The presence of U.S. troops in Pakistan, if publicized, could also undermine a Zardari government that’s taken brave risks to help Washington.

This is where Mr. Obama’s decision to announce a July 2011 deadline for beginning to withdraw from Afghanistan has been damaging. Various Administration officials have tried to walk back that deadline, but it has played inside Pakistan as further evidence that the Americans will eventually bug out of the region. Pakistan’s military and intelligence services have long hedged their bets by supporting Mullah Omar and the Afghan Taliban in case the U.S. leaves and for fear that India will try to fill any power vacuum in Kabul. Now they have another excuse not to change.

The reality is that the gravest threat to Pakistan comes from Islamic radicals, especially if they are able to survive the U.S. and NATO surge. Their next targets will be Islamabad and Rawalpindi as much as Kabul, London or New York. The U.S. and Pakistan share a common enemy, and Mr. Obama will have to assure the Pakistanis that the American commitment won’t end with some arbitrary withdrawal deadline made to appease the U.S. antiwar left.

(The Op-Ed first appeared in The Wall Street Journal)