This paper is driven by a concern with the permissive doctrine concerning the legitimate use of force that has been articulated and defended by many prominent liberals since 9/11 and the advent of the war on terror. The regimes that scholars such as Anne-Marie Slaughter, David Luban or Michael Doyle propose and in particular that defended by Robert Keohane and Allen Buchanan argue in favour of inter alia humanitarian military intervention (sometimes in the form of the responsibility to protect or R2P), a collective duty to prevent the acquisition of WMD by rogue states, a more general right to preventive self-defence,and even forcible democratisation. The proposed weakening of the constraints on the use of force that is characteristic of the UN Charter and of contemporary just war theory is not merely a reaction to the new insecurities presented by the asymmetries of the war on terror although this is a key motivation. It is not a conservative reaction to the frustrations faced by the liberal west in the UN or an endorsement of the Bush Doctrine as formulated in the US National Security Strategies of 2002 and 2006.. Rather it is presented as an articulation of the just defence of basic human rights. I argue that the ‘social moral epistemology’ that underpins Buchanan’s assertive liberalism is the critical heart of his challenge to just war theory but argue that a developed account of ‘institutional moral reasoning’ gives us good reason to respect the just war tradition’s restrictionist attitude to war.
