Recent setbacks in Afghanistan, from the fall of Kunduz to the errant U.S. bombing of a hospital in that city, once again raise a question. Why, after 14 years of American military efforts, is Afghanistan still so fragile?
The country has a democratically elected government widely viewed as legitimate. Poll after poll suggests the Taliban are unpopular. The Afghan army fights fiercely and loyally. And yet, the Taliban always comes back.
The answer to this puzzle can be found in a profile of the Taliban's new leader, Akhtar Mohammed Mansour. It turns out that Mansour lives in Quetta some of the time, The New York Times reports, "in an enclave where he and some other Taliban leaders . . . have built homes."
His predecessor, Mullah Omar, we now know, died a while ago in Karachi. And of course, we all remember that Osama bin Laden lived for many years in a compound in Abbottabad. All three of these cities are in Pakistan.
We cannot solve the problem of Afghanistan without recognizing that the insurgency against that government is shaped, aided and armed from across the border by one of the world's most powerful armies. Periodically, someone inside or outside the U.S. government points this out.
Yet no one knows quite what to do, so it is swept under the carpet and policy stays the same. But this is not an incidental fact. It is fundamental, and unless it is confronted, the Taliban will never be defeated.
It is an old adage that no counterinsurgency has ever succeeded when the rebels have had a safe haven. In this case, the rebels have a nuclear-armed sponsor.
Pakistan has mastered the art of pretending to help the United States while actually supporting its most deadly foes. Take the many efforts American officials have recently made to start talks with the Taliban.
It turns out we were talking to ghosts. Mullah Omar has been dead for two years, all the while Pakistani officials have been facilitating "contacts" and "talks" with him.
This is part of a pattern. Pakistani officials, from former President Pervez Musharraf down, categorically denied that bin Laden or Mullah Omar were living in Pakistan, despite the fact that former Afghan President Hamid Karzai repeatedly pointed this out publicly. "I do not believe Omar has ever been to Pakistan," Musharraf said in 2007.
The Pakistani army has been described as the "Godfather" of the Taliban. That might understate its influence. Pakistan was the base for the American-supported mujahedeen as they battled the Soviet Union in the 1980s. After the Soviet Union retreated from Afghanistan in 1989, the United States withdrew almost as quickly, and Pakistan entered that strategic void.
It pushed forward the Taliban, a group of young Pashtun jihadis schooled in radical Islam at Pakistani madrassas. ("Talib" means student.) Now history is repeating itself. As the United States draws down its forces, Pakistan again seeks to expand its influence through its long-standing proxy.
Why does Pakistan support the Taliban? Pakistan's former ambassador to the United States, Husain Haqqani, whose book "Magnificent Delusions" is an essential guide, explains that "Pakistan has always worried that the natural order of things would be for Afghanistan to come under the sway of India, the giant of the subcontinent.
The Pakistani army came to believe that that it could only gain leverage in Afghanistan through religious zealots.
Afghanistan's secular groups and ethnic nationalists are all suspicious of Pakistan, so the only path in is through those who see a common, religious ideology."
This strategy is not new, Haqqani points out, noting that funding for such groups began in the mid-1970s, before the Soviets invaded Afghanistan in 1979.
What should the United States do? First, says Haqqani, the U.S. needs to see reality for what it is: "When you are lied to and you don't respond, you are encouraging more lies."
He argues that Washington has to get much tougher with the Pakistani military and make clear that its double-dealing must stop. To do this would be good for Afghanistan and stability in that part of the world, but it would also be good for Pakistan.
Pakistan is a time bomb waiting to explode. It ranks 43rd in the world by the size of its economy, according to the World Bank, but has the sixth largest armed forces. It has the fastest-growing nuclear arsenal, and the most opaque. It maintains close ties with some of the world's most brutal terrorists.
Its military consumes 26 percent of all tax receipts by some estimates, while the country has 5.5 million children who don't attend school (the world's second highest number).
As long as this military and its mindset are unchecked and unreformed, the United States will face a strategic collapse as it withdraws its forces from the region.
Fareed Zakaria hosts CNN's "Fareed Zakaria GPS," and makes regular appearances on shows such as ABC's "This Week" and NBC's "Meet The Press." He has been editor at large at Time magazine since 2010, and spent 10 years overseeing Newsweek's foreign editions. He is a Washington Post (and internationally syndicated) columnist. He is author of: "The Post-American World." For more of Fareed Zakaria's reports, Go Here Now.