This month marks the 15th anniversary of the start of the U.S. war in Afghanistan. Rather than ending the war before he leaves office, as he had promised, President Barack Obama is escalating America's involvement in the war.

The Pentagon has deployed special operations troops to support the Afghan army and, for the first time since 2006, has sent B-52 bombers back to Afghanistan to bomb Taliban insurgents. While the bombing has temporarily slowed the Taliban offensive in Helmand province, strategic bombing is unlikely to defeat a resilient guerrilla force that is steadily gaining ground on an Afghan government weakened by corruption and infighting.

America's failure to defeat an insurgency over 40 years ago may offer some lessons that could be applied to the current conflict in Afghanistan. In Cambodia, a brutal guerrilla army had expanded in the countryside and was gradually closing in on the U.S.-backed government in the capital, Phnom Penh. President Richard Nixon and Secretary of State Henry Kissinger believed that B-52 carpet-bombing would defeat the insurgents.

Instead, it bred insurgents. Between October 1965 and August 1973, the U.S. dropped 2,756,941 tons of bombs on Cambodia, more than Allied forces dropped on all of Europe during World War II. At the beginning of the secret U.S. bombing campaign in 1965, there were fewer than 10,000 members of the Khmer Rouge, mostly former university students. By the spring of 1975, their ranks had swollen to 700,000.

I spent the last two months of the war in Cambodia researching the conflict and left on April 12, 1975, along with U.S. Ambassador John Gunther Dean. Before arriving in Cambodia, Dean had been the chargé d'affaires of the U.S. embassy in Laos, where, as a relatively junior foreign service officer, he negotiated a peace agreement between the U.S.-backed government and Lao communists.

Dean thought he was sent to Cambodia with a similar mandate, but soon realized that his boss Kissinger was not interested in negotiations. Soon after arriving in Phnom Penh at the end of March 1974, Dean began sending cables to the State Department urging negotiations with Prince Norodom Sihanouk, the titular head of state of the Government Royal d'Union Nationale de Kampuchea, which was dominated by the insurgents.

Dean proposed an international Cambodian peace conference to form a coalition government. One thousand combatants a day were being killed or wounded in the war and half the population had been displaced. Dean saw a coalition government likely to be dominated by the communist insurgents as the only alternative to the inevitable collapse of the U.S.-backed government, which he said would be "turning defenseless Khmers over to the Khmer Rouge." In an interview achieved at the Carter Library in Atlanta, Dean said, "My position, starting in 1974, and it got shriller as we came towards April of 1975, was that a bad solution is better than a human tragedy."

Dean is now 90 and lives in Paris. He went on to be U.S. ambassador to Denmark, Lebanon, Thailand and India. In a 2007 interview with the Voice of America, he offered advice to policymakers dealing with conflicts in the Middle East. "Sooner or later, the opposition is going to get closer and closer, and militarily it is time to sit down and negotiate," he said. "It was the refusal to find an alternative to military solutions which was the great drama in Cambodia."

In Afghanistan, as in Cambodia, bombing has hardened and radicalized the insurgents. Dean hoped that including elements of Prime Minister Marshal Lon Nol's government, as well as religious leaders, in a coalition government would countervail the power of the Khmer communists and force them to temper their extremist ideology. The Taliban are a rural insurgency and incorporating them into a coalition government would expose them to huge changes that have taken place in Afghanistan's cities in the past 15 years.

By the time Dean arrived in Phnom Penh in early 1974, it may have been too late for a coalition government, for the communists were close to a military victory. In Afghanistan, it is also late in the game, but the Taliban may still agree to a political settlement. They have been fighting for 15 years and worry that the United States and the Afghan army will prevent them from ever taking over all of Afghanistan.

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The Taliban resent being dependent on Pakistan, where they and their families have lived since 2001. Moreover, the Taliban leaders have been feeling pressure this year from the Islamic State group, which has established a foothold in eastern Afghanistan.

The next U.S. president should urgently press for negotiations between the Afghan government and various Taliban factions to reach a power-sharing agreement that would bring members of the Taliban into a coalition government. The U.S. should also convene an international conference to press Pakistan and other neighboring countries to respect the neutrality of Afghanistan; otherwise foreign powers will continue to support proxies inside Afghanistan and undermine any peace agreement.

The United States went into Afghanistan in 2001 to take out al-Qaida. Before withdrawing the last combat forces, the U.S. should extract a commitment from the Taliban never to allow Afghanistan to be used for transnational terrorist activities.

Obama realizes there is no military solution to the war in Afghanistan. As recently as June, he said, "The only way to end this conflict and to achieve a full drawdown of foreign forces from Afghanistan is through a lasting political settlement between the Afghan government and the Taliban." Yet once again the Pentagon is ramping up the war.