LONDON (Reuters) - The fate of Afghanistan after a 20-year war led by the United States means that the West's resolve is now perceived as weak by major adversaries such as Russia, British Defence Secretary Ben Wallace said on Thursday.

       The war in Afghanistan has cost several hundred thousand lives and trillions of dollars but the Taliban are now back in power, and the West's leading powers are scrambling to evacuate their diplomats and Afghan staff from Kabul airport.

       "What I'm uncomfortable with is that we have a world order now, where resolve is perceived by our adversaries as weak, the West's resolve," Wallace told BBC TV.

       "That is something we should all worry about: if the West is seen not to have resolve and it fractures, then our adversaries like Russia find that encouraging," Wallace told LBC radio.

       Britain fears the Taliban's return and the vacuum left by the West's chaotic withdrawal will allow militants from al Qaeda to gain a foothold in Afghanistan, just 20 years after the Sept. 11, 2001 attacks on the United States.

       (Reporting by Sarah Young and Guy Faulconbridge; Editing by Kate Holton and Michael Holden)