Even after 12 years of war US has failed to defeat Afghan Taliban who are fighting a Guerrilla war. With the announcement of US withdrawal a country which is really worried is India. India has invested billions In Afghanistan for its own interests to strengthen pro Indian and Anti Pakistan forces. In case of US Withdrawal all this Indian Investment may go in vein.
There will be a proxy war between India and Pakistan in Afghanistan after American Withdrawal. India will want its friends In power like Northern Alliance. Indian is also trying to create trouble in Balochistan and KP through Afghanistan which will not be possible without a India friendly Govt In Kabul. On other hand Pakistan will want those elements which are not Anti Pakistan. It will help to bring Peace In Pakistan and end terrorism.
Only time will tell what will be the situation after America leaves Afghanistan. When Soviet Union left Afghanistan there was a huge civil war and this time result can be the same.