The permissible use of defensive force is a central tenet of the traditional legal and philosophical justification for war and its practice. Just War Theory holds a nation’s right to resist aggressive attack with defensive force as the clearest example of a just cause for war. Just War Theory also stipulates norms for warfare derived from a conception of defensive force asserted to be consistent with the moral reality of war. Recently, these aspects of Just War Theory have been criticized. David Rodin has challenged the status of national defense as an uncontroversial just cause. Jeff McMahan has charged that Just War Theory’s norms that govern warfare are inconsistent with the norms of permissive defensive force. In this thesis I defend the status of national defense as a clear case of a just cause. However, my defense may require revision of Just War Theory’s norms that govern warfare.