Afghanistan had a profound effect on British military culture. They were defeated on retreat from Kabul in 1842, a loss so devastating to their collective psyche that it was lamented as "the heaviest calamity that has ever fallen on British arms." The Second Anglo-Afghan War, which came at the height of the Great Game between England and Russia, slightly improved the British posture in the region (and was the inspiration for The Ballad of the King's Jest, a terrific Kipling poem).
Over a century later, all the pomp and majesty of British military history resurrects itself in an old neighborhood. The UK forces fighting in Marjah are legendary light infantry and fusilier formations, rich in both experience and victories. The Royal Welsh regiment fighting alongside U.S. Marines is a direct descendant of the Royal Welch Fusiliers, who took Bunker Hill. The 1st Grenadier Guards, which recently celebrated its 350th anniversary, was decisive at Waterloo and fought in both Gulf Wars. And the Coldstream Guards, most famous for their sentinel duties at Buckingham Palace, include Gibraltar, Waterloo, Boer War, Marne, Somme, Dunkirk, El-Alamein, Salerno, and the Persian Gulf War amongst their battle honours.
So history repeats itself, in a sense. The Empire has faded away, but the formations which built London's yawing imperial expanse are still slugging away in remote corners of the world. The territory has dissipated, but the knowledge and fighting skill remains. It's that experience, amalgamating force and government, which makes Britain such an indispensable ally.