What would Clausewitz say about the war against the Islamic State in Iraq and the Levant (ISIL)? As the Prussian sage and his fellow greats of strategic theory might counsel, America is waging an “unlimited war by contingent” against ISIL. Last year President Barack Obama vowed to “degrade and ultimately destroy” the terror state, yet ruled out more than modest air and sea forces to execute this ambitious mission. U.S. ground warfare was out. That raises the question whether the regional contenders battling the Islamic State — the Iraqi army, the Peshmerga, various militias — together comprise a force capable of winning with aerial fire support. Fitful progress on the ground leaves that question open, the allies’ recent re-conquest of Ramadi notwithstanding. The administration has backtracked from its “no boots on the ground” position since, ordering a “specialized expeditionary targeting force” to Iraq early this month. But with overall troop numbers fixed at 3,550, a token number, the sages might voice bafflement at this misbegotten approach to strategy. You cannot know in advance how many troops it will take to crush your enemies and see them driven before you. Such a strategy is a contradiction in terms that is fated to disappoint. Why? Let us parse the first part of the phrase unlimited war by contingent.