I think it's like a losing bluff in poker.
You really want to give your opponent the idea that you're capable of a very, very wide range of behavior, with some of it less reasonable than others of it.
In reality I don't think there are madmen out there as heads of state. I understand that North Korea and Iran are painted that way. Both are depicted as just waiting to get their hands on nuclear weapons so they can commit national suicide.
That's really nonsense.
The anti-missile shield will only be used after a US first strike, or if the US manages to overthrow the governments. It's not to protect from an Iranian or North Korean first strike. That won't happen.
The madmen are those who believe their own propaganda.
Arf