Official Marine Corps photo by Sgt. Mark Fayloga

According to Sens. John McCain (R-Ariz.) and Lindsey Graham (R-S.C.), the situation in Afghanistan is a “stalemate” that “will require additional U.S. and coalition forces.” The senators cite testimony by the top U.S. commander in Afghanistan, Gen. John W. Nicholson, to the Senate Armed Services Committee in which he said he needed several thousand more troops. There are currently about 8,400 U.S. troops in Afghanistan, plus another 6,300 troops from other countries. So will a few thousand more soldiers—presumably American—make a difference?

The clear answer is no.

The rule of thumb for successful counterinsurgency—largely practiced by the British—is a requirement of 20 troops per 1,000 civilians, which is the standard set in the U.S. Army’s counterinsurgency manual. With a population of about 32 million, that means a force of 640,000 troops would be needed in Afghanistan. (For a sense of scale, consider that the total U.S. Army active duty force is less than 500,000 soldiers). Indeed, you would probably have to combine the whole of the U.S. Army with the entire Afghan army of 183,000 soldiers to meet the requirement. Adding a couple thousand troops to some 15,000 U.S. and coalition forces already in Afghanistan is hardly enough.

To be fair to Senators McCain and Graham, it’s not all of Afghanistan that requires counterinsurgency operations. According to the Special Inspector General for Afghanistan Reconstruction, out of the country’s 407 districts, 133 are contested and another 41 are either under insurgent control or influence. These represent a population of almost 12 million people, which would require 240,000 troops. Even this would be a bridge too far for U.S. and coalition forces. If the entire Afghan army—a little less than 200,000 soldiers—shouldered the bulk of responsibility, it would still require another 25,000 U.S. and coalition troops beyond those currently deployed in Afghanistan.

But counterinsurgency is more than just troop levels. Successful counterinsurgency requires the use of harsh—even brutal—and indiscriminate military force to impose security and order. Again, the British example is a good one, such as when their forces put down the Mau Mau Rebellion in Kenya in the 1950s. The problem with such tactics is that while such action may kill the enemy, it also all too often results in killing innocent civilians, no matter how hard we try to avoid collateral damage. Last year, air strikes caused 590 civilian casualties, including 250 deaths, nearly double the number in 2015 and the highest since 2009. More recently, air strikes in the Helmand province are believed to have killed more than a dozen civilians, mostly women and children.

The inevitable result of this collateral damage is alienation of the civilian population, which makes them more sympathetic to the insurgents. Indeed, this is one of the most important lessons of the last decade and a half.

Most importantly, the threat in Afghanistan doesn’t warrant a continued U.S. military presence.

McCain and Graham believe we must confront the Taliban, al Qaeda, the Haqqani network, and the Islamic State. But the Taliban is largely vying for power in Afghanistan and is not waging a global war against the U.S. Al-Qaeda is certainly a threat within Afghanistan, but does not necessarily pose the same terrorist threat to America as pre-9/11 al Qaeda did. The Haqqani network is nationalist in nature and wants foreign troops to withdraw from Afghanistan, and foreign countries to stop interfering in the internal politics of Muslim countries. And ISIS seeks to establish an Islamic caliphate in the heart of the Muslim world.

What they have in common is that they are all Sunni Arab groups seeking to impose their version of Islamic Sharia law in Afghanistan—but ISIS actually opposes the Taliban, al-Qaeda, and the Haqqani network. All four of these groups are internal threats to the Afghan government, but none are direct military or terrorist threats—let alone existential threats—to the United States that require us to spend billions more dollars and risk American lives to defeat them.

Theirs is a war within the Muslim world. In Afghanistan, it is a war that can only be fought and won by the Afghans.

Charles V. Peña is a senior fellow with Defense Priorities and author of Winning the Un-War: A New Strategy for the War on Terrorism. You can follow him on Twitter @gofastchuck.