This volume is the first of two addressing the legal regime governing the use of force during armed conflicts. Traditionally labeled 'Hague Law', today the norms it examines are commonly referred to as 'conduct of hostilities rules'. At the heart of this body of law is the principle of distinction, which requires that civilians and civilian objects be distinguished from combatants and military objectives during military operations. It is the purest expression of the foundational balance between humanitarian considerations and military necessity that has underpinned international humanitarian law since its inception. The essays selected consider the theoretical and practical difficulties of maintaining the balance in the face of evolving means and methods of warfare and competing perspectives as to how it is best achieved. Also addressed is the law governing warfare at sea and in the air. Essays focusing on the former examine early norms and analyze their continuing relevance to today's maritime operations whilst those exploring the latter inject much needed clarity into the subject, an essential task in light of the centrality of aerial warfare in modern combat operations.