Of course there are people in groups who are exceptionsI think if we're talking about the establishment left, that may be true. However, I've also seen plenty (who might regarded as "fringe") strongly oppose the war all along.
When I was growing up, a lot of people were firmly against the Vietnam War (where tens of thousands more Americans died than in Afghanistan, which is a significant difference).
With Afghanistan, it was different, especially after the death of Bin Laden. It seemed to be played up more as a "wolf by the ears" situation. We didn't really want to be there, but letting go seemed an irresponsible option.
There were no communists to fight in Afghanistan. Our enemies were ostensibly fighting for a different ideology and a different cause, thus making the battle for hearts and minds a completely different kettle of fish.
Ever since WW2, both parties favored an internationalistic foreign policy; the isolationists had been thoroughly ostracized and discredited in the eyes of the American people. The general policy of containment and the philosophy which justified has become a cornerstone in our foreign and military policies - embraced by professionals and experts in those fields, and propagated by both major political parties. Both touted themselves as patriotic and pro-American, different only by degrees. Both believed in American exceptionalism.
to general tendencies. But the left is still pro-war.