A tax on the United States is a tax on its citizens. Its citizens, in turn, overwhelmingly support immediate and total withdrawal from the war effort in Afghanistan. So a tax on the U.S. Would only have the impact of imposing further additional costs on U.S. citizens who have already unwillingly paid for 10 years involvement in the war.
While I understand that we needed to help this other country, things have changed and need to be improved in this manner. We are pouring a lot of money into another country with nearly no financial recompense. I think a better solution would be to tax the Afghanistan citizens, or country, to offset some of the financial burden we have taken on.