Then what leverage might the US apply?
America doesn’t need another front in the war in Afghanistan, so sending in troops to Pakistan is unrealistic and unwanted – both by Pakistan’s government and Americans back home. Doing so would probably only cause the kind of political instability that would benefit the militant Islamists. Formally labeling the country a state-sponsor of terrorism could have the same effect.
Another option, cutting off aid, would be unlikely to change the military’s policies. Though Pakistan’s President Asif Ali Zardari sides with the US in his hostility to the Taliban, the elected government lacks the power to impose its will on the military.
The challenge for US-Pakistani relations is Pakistan’s relationship with India and Islamabad's concerns about Mr. Karzai moving to solidify his relations with India. Yet India and Pakistan have recently made surprising progress in talks centered on strengthening economic ties.
The US could play off that positive momentum and encourage New Delhi and Islamabad to open a dialogue on Afghanistan as well. It won’t be easy, but dispelling Islamabad’s fears about India’s intentions in Afghanistan could go a long way toward correcting the Pakistani military’s strategic calculus. (Many assess that Pakistan maintains some alliance with insurgents as insurance against Indian influence in Afghanistan.)