This is a textbook Big Lie, told often enough that it's now "common knowledge". Even Republicans have to repeat the mantra in order to establish their bonnafides.
The clear implication of "Mistakes were made" is that everything in Iraq is our fault. More specifically, Bush's fault.
The problem is that nobody details what "mistakes" they're talking about. On the very rare occasion when someone does offer a specific critique, then it's often as not contradictory from other critiques. Some say "more troops were needed", some say "less troops were needed", etc. The truth is that nothing we could have done would ever have prevented the current terrorism. Not even the Israelies can prevent terrorism 100% of the time, and they have one of the best militaries and armed citizenry on the planet.
The only truth behind "mistakes were made" is we didn't fight the PR battle as well as the jihadies, and that mistake was not done in Iraq, but in New York and Washington.
The "mistakes were made" Big Lie is as empowering to the left today as any of those who were empowered by slanders against Jews in the 1930's.
The Big Lie works, every time it's tried.
Excellent post, narby.