Britain has 7,800 troops in Afghanistan, the second largest troop commitment after the United States. Its commanders have said that to ease severe strains on Britain’s armed forces they need to take their troops out of Iraq without immediately recommitting them to Afghanistan. They have also said they are reluctant to commit more British troops to Afghanistan unless other NATO nations, including France and Germany, agree to step up their troop levels.

The need to replace the departing British troops near Basra will place new strains on American commanders in Iraq. Since 2003, they have relied on British troops to maintain stability in southern Iraq and to guard the vital overland supply route from Kuwait, past Basra and on to central Iraq, where most of the American troops are based. Now, if the British reports are confirmed, those commanders will have to detach an American force of brigade strength to the south, just as they begin drawing down their own troop levels farther north.

According to The Guardian and The Times of London, the 300 to 400 British service personnel who will remain after the drawdown will mainly assist in the training of Iraq’s armed forces.

The British withdrawal will leave to history the controversy that has surrounded the performance of British troops in the south. From early in the war, American commanders were frustrated by what they regarded as an inadequately robust use of British military force, particularly in confronting the rampant militia loyal to Moktada al-Sadr, the radical Shiite cleric, in Basra and Amara.

At times, strains between American and British commanders were severe, with the Americans admonishing the British for acquiescing in the Sadrist group’s takeover of wide areas of the south when American forces were fighting the Mahdi Army, Mr. Sadr’s ruthless militia, in Baghdad, Najaf and other Shiite population centers. Within 18 months of the invasion, British commanders were complaining privately that the Americans lacked Britain’s colonial experience in countries like Iraq, and that the heavy use of firepower against Mr. Sadr was counterproductive.

The British mantra then, and later, was summarized by a British general in Basra who said that Britain had learned from centuries of ruling occupied countries that “you have to govern people as you find them,” and that Mr. Sadr was a reality who had to be accepted. But as Basra and Amara fell increasingly under the sway of militiamen and criminal gangs, the British approach met with growing criticism from defense experts and opposition politicians at home, who said British forces had abandoned ordinary Iraqis to chaos.

The denouement came in March this year, when units of the American 82nd Airborne Division were flown to Basra to rescue Iraqi troops floundering in an ill-prepared bid to drive the Mahdi Army from the city.