There are, in fact, many reasons why Iraq is nothing like Vietnam or any other U.S. experience, but both sides in the American debate over the war have chosen to ignore them. For the antiwar left, Iraq has always recalled the great American trauma of Vietnam, a misguided war of choice that ended badly after a decade of pointless savagery; for the war's advocates on the right, Iraq recalled the great American triumph of rebuilding postwar Japan and Germany. Yes, it is hard to imagine that they were serious, but it wasn't simply PR, either — some of the policy documents used by the U.S. occupation administration in Baghdad were based on policies used in the Allied occupation of Germany.
That comparison obviously looks plain silly, now, so instead we are left with Vietnam — albeit different interpretations of Vietnam. As Professor Juan Cole points out, Bush is probably relying on a hawkish view that while the Tet Offensive was a major military defeat for the Viet Cong, the spike of violence it brought may have struck a crippling political blow at the American public's will to fight the war. As Cole notes, the irony is that the upside of Tet may not be the first thing that comes to mind for Americans when their President compares Iraq to Vietnam — it may be more likely to confirm the belief that Iraq is another quagmire in which the U.S. can't win.
What both Bush and Friedman fail to see is that the catastrophe created by the U.S. invasion is a product of Iraq's own history, culture and composition, and experience of previous invasions — and of the failure of the U.S. leadership to grasp those specifics. It has nothing to do with American experiences elsewhere, and in fact continuing to view events there through the Vietnam prism may have actually contributed to the problem.