Wars always gain in unpopularity when they are not very quick and when the death and casualty count begin to rise. After more than half a decade in Iraq, both of these realities along with a rising economic cost have helped to create an unfavorable perception of the war. At the same time, there are many who feel that the basis for entering the war was fraudulent and not effectively nor honestly conveyed to the American public. The creation of the evidence which presented Iraq to have an abundance of weapons of mass destruction or the notion that then- leader Saddam Hussein had designs to attack the United States, as well as the post- Hussein leadership that has not been entirely forthcoming with America have all helped to create an unfavorable image of the war. At the same time, the economic crisis that is being felt in so many parts of the nation has helped to create a conflicting view of the war throughout the nation.
I think that we are divided on the war Afghanistan even when Bush is not around anymore because the war itself has so many problems. I think that it really seems that we are involved in a war that cannot be won and that our "partners" in this war (Karzai's government) are not really good people that we would want to support.
So it looks like we are wasting our time and liberals think that Obama has sort of betrayed them by continuing Bush-type policies. So I think that many liberals think that this war is being fought when there is no real way to win just because we are too stubborn to leave.
There are those who supported the war in the beginning as a necessary war whether WMD's were found or not. There are those who supported the President no matter what his decisions were and defended the war as a call to patriotism, as something that had already been done, so we should finish the job.
The immense cost has been one reason Americans have soured on the war. Paul Wolfowitz, an adviser to the President suggested the entire war would only cost $1.5 billion and that the oil revenues would pay for it. It has since cost over $700 billion.
The pace of casualties in 2004, then 2006 and 2007 turned some against the war, with 30,000+ wounded and 4300+ dead, some thought the cost was too high. Some of those same people have changed their minds again now that the violence has died down so dramatically.
For many, it was a divisive war based on whether you supported President Bush or didn't, that is, it was a partisan divide as well.