WMD was the only way that the UN obligation for a "Threat to Peace and Security" could be fulfilled. Just wanting to change the regime (in itself very legitimate) is not allowed under the non-intervention rules of the international legal system. So therefore they went on the UN/WMD-tour to war. It would have been better if the UK and USA had really prepared the country for democracy. They chose however to ignore things like the formed government in exile (by all the opposition groups) and wait till the 'Iraqi street' talked.... well it has rather violently.