LAHORE, Pakistan — A British general will command American troops for the first time since World War II when NATO takes charge of the mission to pacify southern Afghanistan today.

Lieutenant General David Richards, Britain’s most experienced officer in developing world arenas, assumes control of a merged NATO and American force that will grow to 18,000 from 9,000.

It is one of the largest and toughest missions that the alliance has faced, covering six southern provinces and extending its authority to almost all of the country. At a press conference in Kabul on Saturday, General Richards said NATO will bring a new strategy to the fight.

Instead of chasing down the Taliban, NATO forces will garrison key towns and villages. It wants to bolster the weak government of President Karzai and win the support of local people by promoting much-needed development.

The general said he hoped there would be “secure zones” in the volatile south in three to six months.

The direct approach pursued under American command, particularly by British troops, has claimed the lives of some 700 Afghan fighters — more than a third of them Taliban — and 19 Western troops, including six British soldiers. American-led coalition forces and Afghan police killed 20 suspected Taliban on Saturday, following an attempted ambush in the Shahidi Hassas district of Uruzgan province.

Since the Americans launched Operation Thrust two months ago, allied forces have been surprised by the ferocity of the Taliban counteroffensive, while there is growing evidence that more Afghan tribesmen, disillusioned with the lack of jobs and reconstruction and the corrupt government, are supporting the rebels.

General Richards emphasized that the opium trade was to blame for a major part of Afghanistan’s violence. “That very evil trade is being threatened by the NATO expansion in the south,” he said. “This is a very noble cause we’re engaged in, and we have to liberate the people from the scourge of those warlords.” It is unclear to what degree leading drug runners will be targeted.

NATO will control security in 75% of the country — in the west, north, and south — while the American-led coalition still leads the fight in the eastern provinces along the border with Pakistan. In the south, the force will comprise mainly British — there are already 4,300 British soldiers — Dutch, and Canadians.

NATO also will command 13 provincial reconstruction teams and take on more responsibility from the Americans for training the Afghan National Army and police.

The European Union representative in Kabul, Francesc Vendrell, told the Daily Telegraph: “The European countries will need to face the fact that sending forces to the south is going to be dangerous, and I am convinced they are ready to take losses, although we want to minimize them.”

The head of the U.N. mission, Tom Koenigs, told the U.N. Security Council last week that 2,000 Afghans had been killed this year and that there were 54 suicide bombings carried out by extremists. These were unknown in the country until January, when they were introduced after Taliban and Al Qaeda members received training in Iraq.

“The violence is four times what it was in 2005 … at no time since the fall of the Taliban have the prospects for security been more bleak,” Mr. Koenigs said.

NATO also has to deal with the sensitive issue of Pakistan’s support for the Taliban. Mr. Koenigs told the United Nations that “the cross-border character of this insurgency is no longer a matter of debate.”

There is also the danger that just as the Iraq war distracted the West from giving more money and troops to Afghanistan after the defeat of the Taliban in 2001, the crisis in Lebanon will have the same effect.