Fareed Zakaria thinks that the fight against the Taliban is overreach with little result. He likens it to fighting Italy after Germany has been defeated. See "Fareed Zakaria Criticizes 'Disproportionate' Afghanistan War on CNN".
We should also look at the reverse situation. How many Americans would fight a foreign invader? Suppose there was an Anti-China League that started hitting Chinese targets. The Chinese, tired of the "American" terrorism, would invade the U.S. with overwhelming force. Many Americans would resist, even if they did not like the Anti-China League.
For what many call a "Christian" nation, we seem incapable of following the second greatest commandment - do unto others what you would have them do unto you.
Also, why does the Afghan army need so much training when the Taliban has such minimal training? If the U.S. can't beat the Taliban with all of its sophisticated training and weapons, will an Afghan army beat the Taliban with the same sophisticated training and weapons? Either a sufficient number of Afghans are opposed to the Taliban to fight the Taliban on similar terms or too few Afghans are willing to risk their lives against the Taliban.