Ideological or Evidence-based Policy-Making ?

We're regularly told by people who should know (better ?) that evidence-based policy-making has triumphed over ideology-based policy during the New Labour administration of Britain. However, I wonder whether this is true in the case of the UK's current involvement in Iraq. I have my doubts, and not only on this policy issue. Either way, perhaps the law of unintended serious consequences should be consulted in advance of major new policy offensives at home and abroad.