The seemingly universal consensus that existed mere months ago that Afghanistan was a "necessary war" the West could not afford to lose has suddenly evaporated. Prominent liberals, centrists and conservatives are coming out in droves to proclaim our goals too lofty and the chances of success too low to justify the high cost in blood and treasure.

If, as now seems inevitable, we leave Afghanistan without finishing the job-whatever that job might be-there will of course be geopolitical consequences. These are being debated at great length elsewhere. Atlanticists, however, have an additional concern: what will the impact be on NATO?

Shortly after taking office in January 2004, then-NATO Secretary General Jaap de Hoop Scheffer declared "Our first, and immediate priority is to get Afghanistan right. We cannot afford to fail." He continued, "If we fail in Afghanistan-if we do not meet our commitments to the people of that country to help them build a better future-then who will have confidence in us again? Our credibility-as NATO, as the Euro-Atlantic community-is on the line. And credibility is one of our strongest assets. To preserve it, we have no choice but to succeed." He repeated this theme throughout his tenure in office, right up to the end.

Scheffer led our alliance superbly through a very difficult period. His words here were, however, terribly unwise.

Quite naturally, citizens in free societies do not want to send their troops to fight and die unless the cause is just and the danger is enormous. This leads to the unfortunate tendency for democratic leaders to oversell war efforts-and undersell the dangers-to rally public support. Recall George H. W. Bush's touting Saddam Hussein as "Hitler revisited," Bill Clinton's promise that American troops would remain in Bosnia no more than a year, or George W. Bush's cherry-picking the intelligence on Saddam's WMD and warning of "mushroom clouds."

Scheffer strongly believed that victory in Afghanistan was in the national-security interests of the West and, yes, that having committed forces, losing resolve and leaving the job unfinished would hurt the alliance's credibility. And declaring that the fate of NATO was on the line was, for a time, an effective way of rallying support among leaders for a war increasingly unpopular at home.

But the fact of the matter is that NATO went to war in Afghanistan, invoking Article V's declaration that an "attack against one" shall be "considered an attack against them all" in the aftermath of the 9/11 attacks. The Taliban government that sheltered al-Qaeda has been ousted and hundreds of the terrorist group's leaders have been killed. The original mission has long since morphed into an incredibly ambitious nation-building exercise with murky goals.

NATO would never have achieved consensus on undertaking such a mission, even in the emotional wake of 9/11. Why, then, should its future rest on its achievement?

Further, while seemingly axiomatic, the idea that withdrawing from a futile effort will cause serious harm to one's "credibility" has not been borne out by history. The United States-and the United Nations-fought for three years to achieve a stalemate in Korea. After more than a decade of fighting in Vietnam and failing to achieve our objectives, we finally decided to "declare victory and go home." After the Blackhawk-down debacle, we decided warlord hunting in Somalia wasn't worth the effort. Yet we've somehow managed to remain the world's dominant military, political, economic and cultural power.

If nation-states manage to slog on after losing or giving up on wars, why should alliances be any different?

James Joyner is the managing editor of the Atlantic Council.