"You can check out anytime, you can never leave?" Not for the American military. President Karzai may be operating under the belief that Afghanistan is too important for the Americans to up and leave. That the U.S. is bluffing. If so, he really sucks at history.
The first American move after every conflict is to go home. The exception was World War II, where the Americans participated in the occupation of German and Japan (ok, and the Philippines). Vietnam? All we were looking for was a "decent interval" between when the US left and when South Vietnam collapsed. Sure, there was the hope that American air support and other assistance could keep the North Vietnamese from winning, but Congress was not so enthused and limited how much assistance could be given. Afghanistan should not rely on any sunk costs argument since the U.S. spent far more of its blood in Vietnam (almost 25x).
Iraq? Well, as the Iraqis are now telling the Afghans, don't think too much of yourselves. “Don’t be under the illusion that no matter what you do the Americans
are here to stay,” Mr. Zebari told Mr. Karzai. “People used to say that
about the American presence in Iraq, too. But they were eager to leave,
and they will be eager to leave your country as well.” Iraq has been far more important to the US due to its oil, its position relative to Iran and so on. Yet we left.
What Karzai and other Afghans seem to forget is that the U.S. has only cared about Afghanistan when it fit into a larger conflict. The US supported the opponents of the Soviet invasion as part of the Cold War. Once the Soviets left, the US didn't think twice about Afghanistan until 9/11. Ok, that might be an exaggeration--the US might have had a thought or two, but certainly did not care much about Afghanistan.
The on-going intervention? It is not that we care about Afghanistan but that we cared about Al Qaeda and its tendency to set up in failed states. We also cared that instability in Afghanistan might be bad for Pakistan. Now that we realize that Pakistan is a major source of that instability, well, little reason to stick around.
What Karzai and other believers in America's endless devotion to Afghanistan seem to ignore is that whole domestic politics thing where folks are tired of war. They also are missing the conversation where the folks who have the tough argument are the supporters of continued involvement. It is easy in the US to argue that it is time to go. The hard argument is to try to argue that it is worth additional money and lives to hang out in a part of the world that has never been in the American sphere of interests (I need to figure out that argument myself).
Central Asia is almost entirely irrelevant to the US. South Asia? Just preventing a Pakistan-Indian war (particularly now that both have nuclear weapons). Otherwise, these places are peripheral. Sure, more than a decade of war might lead one to believe otherwise. But if Karzai thinks that the US really and truly cares about Afghanistan, then perhaps he is smoking some of the pot that grows so tall in his country.