Campaign promises by both Barack Obama and John McCain include sending more troops to Afghanistan. Obama would provide “at least two additional combat brigades [roughly 10,000 troops] to support our effort in Afghanistan.” McCain, not to be outdone, states: “Our commanders on the ground in Afghanistan say that they need at least three additional brigades ... and our commanders in Afghanistan must get them.” But whether it’s two brigades or three, more troops in Afghanistan is not a good idea.

The first question is: where would troops for Afghanistan come from? The current Iraq deployment has already stretched U.S. ground forces thin. Tours of duty in Iraq were reduced from 15 to 12 months only last spring, more than 5 years into the conflict. Nearly 50,000 soldiers have been forced into an involuntary extension of their military service through the use of stop-loss orders. The Army has had to reduce recruiting standards to meet its goals. Over 250,000 members of the National Guard and Army Reserve have been deployed for Operation Iraqi Freedom and Operation Enduring Freedom—with the vast majority going to Iraq and comprising as much as 40 percent of the total force there.

Obama would find troops by redeploying forces from Iraq as part of a deliberate withdrawal timetable. McCain does not advocate a timetable for withdrawal, but he believes troops will become available based on success in Iraq. This amounts to robbing Peter to pay Paul; if the total number of U.S. troops deployed (regardless of whether they are in Iraq, Afghanistan, or anywhere else in the world except back home—and it is worth noting that Obama’s Iraq withdrawal rhetoric has not been specific about bringing U.S. forces home) remains the same, the strain placed on U.S. ground forces will not be relieved.

The problem with simply redeploying forces is troop rotation. Currently, there are approximately 147,000 Army and Marine Corps soldiers in Iraq and another 34,000 in Afghanistan. The rule of thumb is that for every unit deployed, two more are needed to relieve troops in the field at reasonable intervals. So if the total number of troops deployed between Iraq and Afghanistan (or elsewhere in the immediate region) stays constant, 543,000 total troops are needed to sustain the deployment, which is nearly the size of the total active duty U.S. Army and Marine Corps. Additionally, this does not account for the need to rotate the nearly 50,000 U.S. ground troops deployed in Europe and the more than 35,000 in East Asia and the Pacific.

Even if more troops could be found, another 20,000 to 30,000 is probably not enough. There are currently about 60,000 U.S. and other foreign coalition troops in Afghanistan, which has a population of nearly 32 million people. The historical standard for successful occupation operations is 20 soldiers per 1,000 civilians. If history is a reliable benchmark, 640,000 troops would be needed, though an additional 20,000 to 30,000 might be enough to control the capital city of Kabul and keep Hamid Karzai ensconced there as the mayor.

The inconvenient truth is that much of the violence in Afghanistan has its origins in Pakistan, which has become a sanctuary for both al Qaeda and the remnants of the Taliban. More troops in Afghanistan will not solve the problem.

Finally, and most importantly, there is the questionable wisdom of putting more troops in Afghanistan. If there is a lesson to be learned from Iraq, it is not that the surge is working. Rather, it is that military occupation of a Muslim country is not in the United States’ larger strategic interests. A foreign military force is a rallying cry for jihad, as was the Soviet experience in Afghanistan. And we should not forget that 5,000 U.S. troops in Saudi Arabia became the cause célèbre for Osama bin Laden to gather support for making the United States a target of terrorism on 9/11.

Moreover, military operations by occupying forces have been—and would continue to be—the cause of collateral civilian casualties, however unintentional and despite Karzai’s entreaties to avoid killing innocent civilians. Such incidents breed exactly the kind of anti-American resentment that is the basis for terrorist treats to this country. Every innocent civilian killed is someone’s father or mother, sister or brother, relative or best friend. It is safe to say that they do not harbor any great love for the United States, which makes them easy recruits to the ranks of al Qaeda and radical Islam.