"I don't intend to hurry it," McChrystal told reporters traveling with Defense Secretary Robert M. Gates. "It will take a number of months for this to play out. But I don't think that's necessarily a bad thing. It's more important we get it right than we get it fast."

But McChrystal does not have time on his side. The day before he revealed the Kandahar delay, his boss, Gates, said that the U.S.-led coalition has until the end of the year to show progress in the war and prove to the United States and its allies that their forces have broken a stalemate with the Taliban.

"All of us, for our publics, are going to have to show by the end of the year that our strategy is on the right track and making some headway," Gates said Wednesday during a visit to London to meet with British leaders.

McChrystal said he was confident that his counterinsurgency strategy was bearing fruit and that he would be in position to demonstrate that by year's end. "The perception that the insurgency has momentum is reversing," he said. "Progress won't show every day, but it will show over time."

But much will hinge on the outcome of the Kandahar campaign. Asked whether the delay still left time for a decisive outcome by the end of the year, McChrystal was noncommittal. "It will be very clear by the end of the calendar year that the Kandahar operation is progressing," he said. "I don't know whether we'll know whether it's decisive. Historians will tell us that."

U.S. and NATO commanders began preparing this spring for their campaign to gain control of Kandahar, an operation considered crucial to the success of President Obama's strategy for the Afghan war.

But McChrystal said it was taking longer than expected to gain the blessing of local tribal leaders -- and Kandaharis in general -- for the operation. He also said commanders needed more time to ensure that the Afghan government could step in after the fighting stops and provide effective public services, which Kandahar has lacked for years.

"When you go to protect people, the people have to want you to protect them," McChrystal told reporters. "It's a deliberate process. It takes time to convince people."

As part of that effort, he said, he will join Afghan President Hamid Karzai on a visit to Kandahar in the next few days to hold "shuras," or council meetings with tribal elders. Karzai, he said, would focus "on all things to improve in Kandahar: security, governance, reducing corruption."

McChrystal did not specify how the timeline of the Kandahar operation would change. But he projected that U.S. and NATO forces in Kandahar would total about 23,300 by the end of August, about three times as many as a year ago.

He also did not say why it was taking longer to lay the political groundwork for the operation. But one problem has been that the Taliban has already responded by assassinating several key local leaders, part of an intimidation campaign to warn Kandaharis against helping the foreigners.

The Afghan government is already officially in charge of Kandahar, the country's second-largest city, but its control is tenuous. The Taliban has steadily made a comeback since the U.S.-led invasion of Afghanistan in 2001.

Another difficulty for U.S. officials has been deciding how to handle local power broker Ahmed Wali Karzai, the president's brother and head of the Kandahar provincial council. Some U.S. agencies, including the CIA, see Ahmed Wali Karzai as an asset, while others view him as a stumbling block who has allowed corruption to flourish on his watch.

McChrystal said the decision to move more slowly in Kandahar was influenced by the experience of U.S. and NATO forces in next-door Helmand province. In February, foreign and Afghan troops led by U.S. Marines took control of Marja and other districts long held by the Taliban. But the effort to install a functioning Afghan government in the wake of the fighting has stumbled and now the Taliban is trying to reassert itself.

"Some of the lessons we learned in Marja is we did very good coordination with the Afghan people, shuras and whatnot, but then as we did it, we found it even more complex than we thought," McChrystal said. "And so we need to educate ourselves from that and do it even better in Kandahar."

Asked whether he considered the Kandahar delay a setback in the Afghan war, Anders Fogh Rasmussen, NATO's secretary general, said the difficulties actually indicate progress in the overall war effort.

"I foresee a very tough time in the coming weeks and months, because we are now targeting what I call the Taliban heartland in Helmand and Kandahar," he said at a news conference here. "But that's exactly our goal."