Nearly 20 years after invading Afghanistan to oust the Taliban and hunt down al-Qaida, the U.S. military has vacated its biggest airfield in the country, advancing a final withdrawal that the Pentagon on Friday said will be completed by the end of August. President Joe Biden had instructed the Pentagon to complete the military withdrawal by Sept. 11, the 20th anniversary of the terrorist attacks on the United States, but the Pentagon now says it can finish the drawdown a little earlier. In fact, the drawdown is already largely completed and officials had said it could be wrapped up this weekend. But a number of related issues need to be worked out in coming weeks, including a new U.S. military command structure in Kabul and talks with Turkey on an arrangement for maintaining security at the Kabul airport, and so an official end to the pullout will not be announced …