So far, President Obama has failed to answer some basic questions on Afghanistan. Until he does, weighing how many troops to add is like putting the cart before the horse.

President Obama must first clarify the mission: what does he want to accomplish in Afghanistan? So far, he’s talked about a number of missions -- to defeat and destroy Al Qaeda and/or the Taliban, to win the hearts and minds of the Afghan people, and to buy time for the Afghans to build a national army and police force. These are all different missions, that require different – and sometimes conflicting -- military strategies and resources.

Is Our Goal to Destroy Al Qaeda? Well, Al Qaeda isn’t in Afghanistan anymore; we drove its ragtag remnant out in December 2001 when it decamped across the Tora Bora Mountains into the tribal regions of Pakistan.

Is Our Goal to Prevent Al Qaeda from returning to Afghanistan? Yes, but can that be accomplished short of remaking Afghanistan? Do we really need 100,000 American troops and over $100 billion a year to keep a few thousand Al Qaeda from walking across the border? Should we continue to funnel our resources through a corrupt and incompetent President Karzai who, after eight years, is still more like the mayor of Kabul than the president of all Afghanistan? Or can we work directly with the powerful tribal chieftains and regional leaders? Would they be willing to guarantee that their areas remain Al Qaeda-free zones in exchange for generous payments? Afghanistan is a place where it’s often cheaper and more effective to buy friends than kill enemies.

Is Our Goal to Destroy Al Qaeda Worldwide? If so, isn’t that hampered by tying down the majority of our military assets in Afghanistan while Al Qaeda grows its footholds in Pakistan, Yemen and Somalia?

How Does Afghanistan Fit Into Our Wider National Security Goals? Our number one national security priority should be keeping nuclear weapons out of the hands of terrorists. To do that we, must destroy Al Qaeda, not just in Afghanistan, but worldwide. At a minimum, we must keep them away from nuclear weapons. Nation building, spreading democracy, and winning the hearts and minds of people throughout the world might be noble goals, but they do not necessarily help us destroy Al Qaeda.

Does Afghanistan Actually Distract Us From This Goal? While we focus our efforts on Afghanistan, Al Qaeda is now headquartered in nuclear-armed Pakistan, closely allied with the Pakistani Taliban, fighting a regional civil war and threatening to destabilize that government. Even if we ‘win’ in Afghanistan, and Pakistan’s government falls, it’s just a few short steps before their nuclear arsenal is in the hands of terrorists.

Does Adding Troops In Afghanistan Help Pakistan In Its Fight Against the Taliban? Perhaps, but not necessarily. And, according to Pakistani leaders, the recent American decision to pull out of the Afghan border regions and consolidate forces in the cities does just the opposite. While American forces had provided a bulwark on the border, their departure gives the Taliban a back door escape route from the fighting in South Waziristan. So, even if the Pakistani Army defeats the Taliban in Waziristan, a remnant will no doubt migrate back into Afghanistan, thus starting the cycle all over again.

Is Our Ability to Act Decisively In Crises Around the World Compromised If We’re Tied Down In Afghanistan for the Foreseeable Future? Gen. McChrystal’s report makes clear that even adding troops doesn't guarantee "victory." What it does make clear is our commitment to Afghanistan will be expensive and long term. When I visited Afghanistan in May, everyone I interviewed --Afghan politicians, U.S. military officers, NATO commanders, NGO workers -- spoke in terms of years, if not decades. What if crises develop with Iraq? Iran? Israel? Pakistan? Russia? The Persian Gulf? North Korea? China? Taiwan? Will we have the military capacity and economic resources to deal with them as well?

Unless President Obama can answer all these questions, merely throwing more troops at Afghanistan won’t work. And beginning Tuesday, Afghanistan becomes Obama’s war.

Kathleen Troia “KT” McFarland served in national security posts in the Nixon, Ford and Reagan administrations. She wrote Secretary of Defense Weinberger’s November 1984 "Principles of War Speech" which laid out the Weinberger Doctrine. She is a senior adviser to the Foundation for the Defense of Democracies and a frequent contributor to the Fox Forum.