We've been at war with the Sharia-fascists for decades, though most Americans didn't bother to take notice pre 9/11. On 9/11 the war became impossible for us to ignore, so we started acting like a nation at war. We resolved ourselves for a long, drawn out, grinding process to make the world safer for us, and we went to it. We made plenty of mistakes along the way (hey, it's war, show me any war that was carried off perfectly), but we also made a helluva lot of progress.
Inevitably, our resolve has waned. Now it seems that after only 9 years it is once again becoming possible to ignore the ongoing and unresolved war we're in. People are starting to forget that when you don't go to the war, the war comes to you.
If we leave Afghanistan without a stable, allied government in place, we're very likely to find that the region reverts back to a staging and launching ground for attacks against us. Eventually one of the attacks will be wildly successful, killing an "unignorably" large number of our citizens, and we'll start all over again.
This war didn't start with our entry into Afghanistan, and it won't end with our exit from Afghanistan. If we withdraw now, the war won't end, it'll simply shift from rural Afghanistan back to the streets of New York (or LA, or Dallas, or...). We'll still be fighting them, only now it'll our unarmed/untrained/unequipped civilians rather than our mighty warriors who have to get up close and personal with the enemy.
I'm not sure what the solution is, other than to fight to win and not quit until we do. That option just doesn't seem viable, we apparently don't have the fortitude to carry it out. C'est la vie.