If the aphorism holds—the guerrilla wins if he does not lose—the Taliban are winning and America is losing the war in Afghanistan.

Well into the eighth year of war, the Taliban are more numerous than ever, inflicting more casualties than ever, operating in more provinces than ever and controlling more territory than ever. And their tactics are more sophisticated.

Gen. Stanley McChrystal calls the situation “serious.” Chairman of the Joint Chiefs Adm. Michael Mullen calls it “serious” and “deteriorating.”

President Obama thus faces a decision that may decide the fate of his presidency. For if the situation is grave and deteriorating, he cannot do nothing. Inaction invites, if it does not assure, defeat.

Does he cut U.S. losses, write off Afghanistan as not worth any more American blood and treasure, and execute a strategic retreat?

Or does he become the war president who sends McChrystal the scores of thousands of U.S. troops necessary to stave off a defeat for all the years needed to conscript and train an Afghan army that can and will defend the Kabul regime and pacify the country?

Afghanistan is being called Obama’s Vietnam.

It could become that, and bring down his presidency as Vietnam brought down Lyndon Johnson’s. But Afghanistan is not yet Vietnam in terms either of troops committed or casualties taken.

The 68,000 Americans who will be in Afghanistan at year’s end are an eighth of the forces in Vietnam when Richard Nixon began to bring them home. Vietnam cost the lives of 58,000 Americans. The Afghan war has cost fewer than 1,000. U.S. casualties in Afghanistan are as yet only a fifth of the U.S. losses in the Philippine Insurrection of 1899-1902.

If we compare Afghanistan to Vietnam, we are about in 1964, when the Tonkin Gulf Resolution was passed and the bombing of the North began, or December 1965, when the Marines came ashore at Danang.

Obama can still choose not to fight this war.

But should he so choose, he will be charged by Republicans and neoconservatives with a loss of nerve, with having cut and run, with having lost what he himself has repeatedly called a “war of necessity,” with having abandoned the noble cause for which many of America’s best and bravest have already paid the ultimate price.

And it needs be said: The consequences of a U.S. withdrawal today would be far greater than if we had never gone in, or had gone in, knocked over the Taliban, run al-Qaida out of the country, gotten out and gone home.

Instead, we brought NATO in, put tens of thousands of troops in and declared our determination to build an Afghan democracy that would be a model for the Islamic world, where women’s rights were protected.

After inviting the world to observe how the superpower succeeds in taking down a tyranny and creating a democracy, we will have failed, and we will be perceived by the whole world to have failed.

While there was no vital U.S. interest in Afghanistan before we went in, we have invested so much blood, money and prestige that withdrawal now—which would entail a Taliban takeover of Kabul and the Pashtun south and east—would be a strategic debacle unprecedented since the fall of Saigon.

But what if Obama approves McChrystal’s request and puts another 20,000 to 40,000 U.S. troops into the war?

Certainly, that would stave off any defeat. But what is the assurance it would bring enduring victory closer? The Taliban have matched us escalation for escalation and are now militarily stronger than at any time since the Northern Alliance, with U.S. air support, ran them out of Kabul.

About the political consequences of escalation, there is no doubt.

Obama would divide his party and country. His support would steadily sink as the roll call of U.S. dead and wounded inexorably rose. He would watch as the NATO allies moved toward the exit and America was left alone to fight alongside the Afghans in a seemingly endless war.

Consider. If there were no Americans in Afghanistan today, and the Taliban were on the verge of victory, how many of us would demand the dispatch of 68,000 troops to fight to prevent it? Few, if any, one imagines.

What that answer suggests is that the principal reason for fighting on is not that Afghanistan is vital, but that we cannot accept the American defeat and humiliation that withdrawal would mean.

Thus Obama’s dilemma: Accept a longer, bloodier war with little hope of ultimate victory, a decision that could cost him his presidency. Or order a U.S. withdrawal and accept defeat, a decision that could cost him his presidency.

In such situations, presidents often decide not to decide.

Harry Truman could not decide in Korea. LBJ could not decide in Vietnam. Both lost their presidencies. Ike and Nixon came in, cut U.S. losses and got out. The country rewarded both with second terms.