Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President George W. Bush from 2001 to 2002, he is the author of six books, including "Comeback: Conservatism That Can Win Again," and is the editor of FrumForum.

Washington (CNN) -- The killing of Osama bin Laden raises many haunting questions. Here's the most important:

Has our mission in Afghanistan become obsolete?

To think through that question, start with a prior question: Why did we remain in Afghanistan after the overthrow of the Taliban?

The usual answer to that question is: To prevent Afghanistan from re-emerging as a terrorist safe haven.

There have always been a lot of problems with that answer. (For example: Does it really take 100,000 U.S. troops, plus allies, to prevent a country from becoming a terrorist safe haven? We're doing a pretty good job in Yemen with a radically smaller presence.)

But this week, we have exposed to sight two huge problems with the usual answer.

1. The world's most important terrorist safe haven is visibly not Afghanistan, but instead next-door Pakistan.

2. Because the U.S. presence in Afghanistan requires cooperation from Pakistan, the Afghanistan mission perversely inhibits the United States from taking more decisive action against Pakistan's harboring of terrorism.

Here's a very concrete example. Through the 2008 presidential campaign, candidates John McCain and Barack Obama tussled over the issue of direct anti-terrorist action inside Pakistan. On February 20, 2008, McCain called Obama "naive" for suggesting that he might act inside Pakistan without Pakistani permission.

In retrospect, McCain's answer looks wrong. But think about why McCain said what he did. He knew that acting in a way that offended Pakistan would complicate the mission in Afghanistan. The United States looks to Pakistan to police the Pashtun country on the other side of the Afghanistan-Pakistan border. Guerrilla wars become much harder to win if the guerrillas are allowed sanctuary across an international border. So if the mission in Afghanistan is the supreme priority, then acting in ways that offend Pakistan must be avoided.

But this thinking leads to an upside-down result: In order to prevent Afghanistan from ever again harboring a potential future bin Laden, we have to indulge Pakistan as it harbors the actual bin Laden!

Some Democrats have retrospectively seized on McCain's upside-down logic as proof that candidate Obama was "right" in 2008. I was a guest on the Bill Maher program on HBO on Friday night where he insisted on this point.

But, of course, President Obama has made decisions that have aggravated the upside-down problem. By inserting so many additional U.S. forces into Afghanistan, he has made the United States more dependent than ever on Pakistan -- with the result that even after finding and killing Osama bin Laden in the heart of Pakistan's national security establishment, the Obama administration is reluctant to challenge Pakistan publicly or even privately.

Think now: What would our policy in South Asia look like if we had a much smaller mission in Afghanistan? Perhaps 20,000 U.S. and allied troops on a security assistance mission rather than 100,000-plus on a combat mission?

By emancipating itself from dependence on Pakistan, the United States would gain scope to focus on the most vital questions in the U.S.-Pakistan relationship, such as:

• How confident do we feel that the people who sheltered bin Laden do not also control Pakistan's nuclear force?

• If we do not have confidence in the people who control Pakistan's nuclear force, what plans do we have to disable that nuclear force?

• Why wasn't Pakistani nuclear scientist A.Q. Khan, the Johnny Appleseed of nuclear proliferation, delivered to U.S. custody?

• Pakistan has a long history of not only harboring anti-U.S. terrorism, but actively promoting and supporting terrorism against India. Why is Pakistan not listed alongside Iran as a state sponsor of terror?

• Why is Pakistan receiving U.S. military aid?

• Why does Pakistan have the benefit of a trade and investment agreement with the United States?

Instead, even now -- even now! -- we're told that Pakistan is just too important to permit the U.S. to act on its stated doctrine--articulated by George W. Bush's administration and not repudiated by Obama's: "Those who harbor terrorists will be treated as terrorists themselves." So long as we remain in Afghanistan, that statement remains true. The question is, shouldn't we be taking now the steps to render the statement less true?

The less committed we are to Afghanistan, the more independent we are of Pakistan. The more independent we are of Pakistan, the more leverage we have over Pakistan. The more leverage we have over Pakistan, the more clout we have to shut down Pakistan's long, vicious, and now not credibly deniable state support for terrorism.

The opinions expressed in this commentary are solely those of David Frum.