“I really am not going to get involved in a discussion about the legal position of the Iraq war. I am not the person to do that because I am not sufficiently impartial as a lawyer about this, because it’s a matter that is of interest to the person that I am closest to in the world.”
Being impartial can mean to treat everyone fairly when there is an argument or disagreement. It could mean that you listen to both sides of an argument before making a decision about what to do even though one of the people involved is your friend. To be impartial you need to put any biases or prejudices to one side.