Poland's Lesson #1 for Iraq: "Make sure all the Nazis are dead and under no circumstances allow them places in the new government." ;)
If you are talking about the ex-Baathists our mistake in the first place was firing them all from their administrative jobs (and the military) and basically left half a million angry unemployed Sunnis with no hope for employment in the new Iraq. And, in comes Zarqawi, Syria, and Saddam's generals with billions of dollars telling these people that they can get 500$ just to place a bomb by the side of the road.
We are now begging the Iraqi government to let us bring these same people in to the military and the government.
There is an old saying, keep your friends close and your enemies closer. Patton understood that quite well in how he dealt with post WW2 Germany.