I'm sure it was just to see if the US used nuclear missiles. /s
or biological weapons :)
By the way the Top Secret and Personal letter carried the highest level of secrecy in Saddam regime.
Sarcasm aside, I wonder if they truly were this ignorant. When we invaded, the Iraqis thought that our Javelin Medium Antitank Missiles were tactical nukes due to the degree of destruction that they caused to T-72's. They also thought that our body armor was air conditioned, that our water made us super human, and that our armored vehicles had showers inside.