The notion that the British army is in Afghanistan to seek revenge for 19th century defeats is of course absolutely grotesque, but that is not the point. The point is that ordinary Afghans do indeed believe this – and the British security establishment ought to have known that they would. That we did not know this is a shattering illustration of the fact that while British policy is in the end powered by sublimated imperial nostalgia, most of the really valuable practical memories and lessons of empire have long since been forgotten.-- Anatol Lieven.
John Lanchester notes a key factor in British involvement in the most recent Afghan war:
After Kosovo, Blair’s world view solidified. He became more certain about his judgments, and more willing to use force—as he did, again successfully, and again without a U.N. mandate, in Sierra Leone, where, in 2000, British troops imposed a ceasefire on a bloody civil war. Given this growing sense of conviction—of the need to take a “fronting-up, out-there leadership position and stake it all on winning”—it was no surprise that Blair was strongly on the side of going to war in Afghanistan and then, much more controversially, in Iraq.