In the short run, however, Ambassador William Patey told the BBC he expects Afghanistan to be "a fragile, unstable state with weak institutions and higher levels of corruptions than is healthy for any country."
Patey is retiring from his post as ambassador to Afghanistan, which he assumed in 2010.
In the BBC interview, Patey said the country's "training wheels" will be removed in 2014 when most British and U.S. troops are to leave. He said he is "cautiously optimistic" about the future if Western countries continue to give Afghanistan aid.
"Afghanistan, having suffered decades of conflict, is not about to become, after one decade of international help, a modern liberal democracy," Patey said.
The biggest mistake made by countries in the NATO alliance was to believe, prematurely, that victory had been won, he said.
He finds one hopeful sign in reports that Taliban leaders have decided they cannot win a military victory in Afghanistan and are prepared to negotiate.