Domestic public opinion is frequently and correctly described as a crucial battlefront in the war in Afghanistan. Commentary by media and political figures currently notes not only the falling support for the war in the United States but also in many of its key allies in Europe and elsewhere, making it all the more difficult for the Obama administration to secure the help it believes it needs to bring the war to a successful conclusion. This study is an extensive examination of the determinants of domestic support for and opposition to the war in Afghanistan in the United States and in five of its key allies--the United Kingdom, France, Germany, Canada, and Australia. Tracing the trajectory of public opinion on the war from the original invasion in 2001 to the fall of 2009, this paper concludes that the combination of mounting casualties with a declining belief that the war could be won by the Coalition is the key factor driving the drop in support. Other factors, such as the deployment of numerous and shifting rationales by the political leadership in various countries, and the breakdown of elite consensus have played important but secondary roles in this process.