Afghanistan will hold a runoff election after all. After weeks of intense speculation, fueled by allegations of fraud in the first round of the presidential ballot, election monitors have concluded that incumbent President Hamid Karzai did not win an outright majority.

Holding another vote will be difficult, given the approach of winter and the Taliban's efforts to disrupt the vote. But Afghanistan must have a legitimate government if there is to be any hope of peace in that embattled country.

The first round in Afghanistan's presidential election was held Aug. 20. Mr. Karzai, president since 2001, won the ballot, claiming 54.6 percent of the votes cast in the 36-candidate race. According to Afghan law, an outright victory — with more than 50 percent of the votes cast — would have eliminated the need for a runoff. The top contender, Mr. Abdullah Abdullah, a foreign minister in a previous Karzai government, challenged the results, claiming widespread fraud.

Amid protests, the findings of election monitors, which concurred with Mr. Abdullah's complaints, were decisive. A United Nations-backed panel threw out nearly 1 million of Mr. Karzai's votes — nearly one-third of the number he received — pushing him below the 50 percent threshold and forcing a runoff against Mr. Abdullah. The process by which the U.N. panel reached its conclusions was accompanied by considerable drama. The No. 2 of the group, an American, resigned complaining that its head, a Norwegian, was ready to turn a blind eye to fraud because he felt Mr. Karzai would eventually prevail and that it was vital to create a new government and end the leadership vacuum created by two months of uncertainty after the vote.

That complaint later appeared unfounded. Or maybe the resignation ruckus shifted the media spotlight. Either way, the mission concluded that Mr. Karzai could not claim an outright win. The findings of the Afghan-led Independent Election Commission, which has the final say on the election outcome, tipped the balance. Unlike the U.N. panel, this group comprises an Afghan majority — most of whom support the president. There had been doubts about its readiness to challenge Mr. Karzai's win. While the commission had "some reservations" about the fraud rulings, it agreed on the need for a runoff because of "time constraints, the imminent arrival of winter and existing problems in the country."

Despite some misgivings, Mr. Karzai accepted the decision. Credit his understanding of the need for a legitimate government in Kabul and the considerable pressure he got from the international community, the U.S. in particular. The election commission says the next round of voting will take place Nov. 7.

Holding another vote will not be easy. Afghan winters are fierce and the country is large, desolate and lacking infrastructure. It took days for some ballot boxes to get to villages; it will be even harder when the snow falls. Moreover, the Taliban remains opposed to the elections as its insurgency strengthens. Turnout in the first round of voting fell below that of the last presidential election as a result of Taliban threats and intimidation. Insurgents cut off the ink-stained fingers of many voters. The number could drop even further if voters fear that their ballots may not count or are at risk of being disqualified again.

An election may not be the ultimate arbiter of who governs Afghanistan. Mr. Karzai is being pushed to accept a national unity government with Mr. Abdullah, who has hinted in recent statements that he is receptive to the idea. Mr. Karzai, however, has said "there is no space for a coalition government in the law." That could be a bargaining tactic or it could reflect his confidence that victory is inevitable for a member of the Pashtun majority.

No matter what the outcome, the results must be respected as legitimate, which begs the question of whether Afghanistan can afford not to end up with a power-sharing deal and some sort of broad-based government. The political situation is deteriorating and foreign support, no matter how large, cannot shore up a government that does not represent the majority of the Afghan people.

The United States has made it clear that it is not prepared to sink more troops and more money into a war if its partner is not accepted as the legitimate authority in the country. Washington has faced that situation before and the result was the debacle in Vietnam. As U.S. President Barack Obama contemplates reinforcing U.S. forces in Afghanistan, the credibility and viability of the Afghan government is an increasingly significant factor in his calculations.

Mr. Karzai is said to have resented U.S. pressure to accept either a runoff election or a power-sharing accord. He believes that Washington wants him out of office. Acknowledging the fraud that tainted the August elections is a first step toward installing a truly legitimate government in Kabul. Failing to do that, Mr. Karzai would risk being ousted — not by the U.S. or his allies but by forces that seek to overturn the Afghan government and return it to Taliban rule.