Clip_152Nearly 15 years after 9/11, the war in Afghanistan is raging and Pakistandeserves much of the blame. It remains a duplicitous and dangerous partner for the United States and Afghanistan, despite $33 billion in American aid and repeated attempts to reset relations on a more constructive course.

In coming weeks, Gen. John Nicholson Jr., the new American commander in Afghanistan, will present his assessment of the war. It’s likely to be bleak and may question the wisdom of President Obama’s goal of cutting the American force of 10,000 troops to 5,500 by the end of the year. The truth is, regardless of troop levels, the only hope for long-term peace is negotiations with some factions of the Taliban. The key to that is Pakistan.

Pakistan’s powerful army and intelligence services have for years given support to the Taliban and the Haqqani terrorist network and relied on them to protect Pakistani interests in Afghanistan and prevent India from increasing its influence there. Under American pressure, the Pakistan Army recently waged a military campaign against the Taliban in the ungoverned border region. But the Haqqanis still operate in relative safety in Pakistan. Some experts say the army has helped engineer the integration of the Haqqanis into the Taliban leadership.

Pakistan’s double game has long frustrated American officials, and it has grown worse. There are now efforts in Washington to exert more pressure on the Pakistan Army. Senator Bob Corker, Republican of Tennessee, chairman of the Senate Foreign Relations Committee, has wisely barred the use of American aid to underwrite Pakistan’s purchase of eight F-16 jet fighters. Pakistan will still be allowed to purchase the planes, but at a cost of $700 million instead of about $380 million.

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Mr. Corker says he would lift the hold on the aid if Pakistan cracks down on the Haqqani network, which he called the “No. 1 threat” to Afghanistan and American troops there.

President Ashraf Ghani of Afghanistan is also getting tougher with Pakistan’s leaders. He courted Pakistan for more than a year in the hopes that the army would bring the Taliban to the negotiating table. But the surge in violence forced him to effectively end negotiations. Last month, he threatened to lodge a complaint with the United Nations Security Council if Pakistan refuses to take military action against Taliban leaders on its soil.

While such pressure makes sense, severing ties as the United States did in the 1990s after Pakistan developed a nuclear weapon is unwise. The two countries still share intelligence, and Pakistan allows American drones to target militant leaders in the border region. Given that Pakistan has the world’s fastest-growing nuclear arsenal, America needs to be able to maintain a dialogue and help Pakistan keep the weapons out of the hands of extremists.

In 2015, more Afghan civilians and troops were killed than in any other year since the Taliban was toppled in 2001. Since taking office in 2014, Mr. Ghani has been a more reliable leader than his predecessor, Hamid Karzai. But his unity government is crippled by political infighting, endemic corruption, a budget crunch and an unsustainable troop casualty rate.

That grim reality presents difficult choices for Mr. Obama, who must decide whether to keep the current troop strength and possibly to change the military’s role to fight the Taliban more directly.

President Obama declared, with undue optimism, more than 16 months ago that “the longest war in American history is coming to a responsible conclusion.” It will be left to his successor to figure out how and whether the Taliban can be lured into political negotiations. That will only happen if the American government finds a way to convince Pakistan to stop fueling the war.