El principio del doble efecto en el ius in bello.

This paper aims to explain how Elizabeth Anscombe understands the principle of double-effect proposed by Thomas Aquinas in order to solve issues such as the legitimacy of killing in self-defense or of defensive war. The basis of this principle in Aristotle's philosophy is analyzed, especially the difference between willing per se and willing per accidens. Finally, some difficulties o f implementing the principle of double effect are highlighted: particularly the distinction between intended and unintended but expected consequences of action, and the importance of distinguishing the harm caused by oneself and the harm caused by somebody else. [ABSTRACT FROM AUTHOR]