WMDs
If anyone bothered to read the actual memo, it is interesting to note that one of the main issues discussed by the British is what would happen in an invasion if Saddam used his WMDs on the invading force.
Considering this was a secret planning meeting, it's going to be damn hard to sell the idea that they were lying about their belief that Saddam had WMD.
In other words, you need to sell people on the idea that planning a war against Iraq when the planners were honestly convinced Saddam had WMD is a problem.
I'm sure the Left will try anyway, though. They pretty much count on people not dealing with original sources.