What is your military education level? If I may ask?
Not a day of service... My family history is storied... Korea, World War II, World War I and the Civil War... So I do pay attention to history.
Interesting piece of history... America still has bases in Germany, Japan, South Korea, Iraq and most of the other places that they have invaded over the past century... Why leave Afghanistan when you can stay, have a base of operations and maintain order? Pakistan and Iran are on the border of this country... Two very dangerous countries that the USA and its allies may need to invade at some point in the future.
Afghanistan is now a bloody mess. Leaving it was not a very good policy strategically, politically or economically. The cost of re-invading both in terms of money and blood will be way more than the cost of staying put.