KABUL, Afghanistan — Prime Minister Blair said yesterday that Western leaders had underestimated how long it would take to win the war on terror as he made his first visit to the Afghan capital, Kabul, amid intense security.

Five years after the Taliban was driven from the city on November 13, 2001, the prime minister conceded that the West had wrongly presumed — when the Taliban fled — that victory was assured. Now, after 18 British soldiers have died at the hands of a resurgent Taliban in the past six months, he accepted that lessons had been learned in both Afghanistan and Iraq.

It was the latest admission by Mr. Blair that the war on terror had not gone to the original American-British plan and is taking far longer, with greater costs, than anticipated.

Mr. Blair, who called on supporters of the war on terror to “rediscover” their self-belief, said Western leaders now realized that the terrorist enemy presented a challenge that it would take “a generation” to face down.

“I think we are wiser now to the fact that this is a generation-long struggle,” he told a press conference with President Karzai. “I am not saying we remain here in the same way [with 6,000-plus British troops] for that length of time. But I am saying that overall, the roots of the Taliban, where they got footholds in a country like Afghanistan, it is going to take time to banish that for good.”

At a joint appearance in Kabul, Messrs. Karzai and Blair said the reconstruction of Afghanistan, on which Britain will spend almost $1 billion over the next three years, was bringing real benefits to the Afghan people.

So fast was the progress in health, education, and transport, Mr. Karzai said, many Afghans felt “life was advancing at a pace we can’t keep up with.”

Kabul he added, already suffered Western-style traffic jams, a sure sign of its advancing economy.

Britain has more than 6,000 troops in Afghanistan, the majority of them now in Helmand province.