By itself, the U.S.-trained and -fielded Afghan army will require $5-$7 billion a year in U.S. support to field an army of 350,000 in a country the size of France. Nothing is less certain.
With major defense cuts in the works, the Pentagon will have insufficient funds to maintain current force levels in the U.S. Army, U.S. Navy, U.S. Marines, and U.S. Air Force. It certainly won't have the wherewithal to fight a two-front war as it did in Iraq and Afghanistan.
Defense budget supplementals throughout the first decade of the 21st century doubled the real costs of defense in a two-war configuration. Taxpayers didn't feel any pain as the real cost of $1.5 trillion ($1 trillion for Iraq, $500 billion for Afghanistan and counting) was simply added to the national debt. Thus, de facto war tax supplementals were never an issue.