However, in the spirit of the game, I will direct you to perhaps the single most influential writer on the laws of war.
From Wikipedia (
http://en.wikipedia.org/wiki/Hugo_Grotius):
Hugo Grotius (also known as Huig de Groot or Hugo de Groot; 10 April 1583 – 28 August 1645) worked as a jurist in the Dutch Republic. With Francisco de Vitoria and Alberico Gentili he laid the foundations for international law, based on natural law. He was also a philosopher, theologian, Christian apologist, playwright, and poet.
De jure belli ac pacis libri tres (On the Law of War and Peace: Three books) was first published by Hugo Grotius in 1625, dedicated to Grotius' current patron, Louis XIII. The treatise advances a system of principles of natural law, which are held to be binding on all people and nations regardless of local custom. The work is divided into three books:
Book I advances his conception of war and of natural justice, arguing that there are some circumstances in which war is justifiable.
Book II identifies three 'just causes' for war: self-defense, reparation of injury, and punishment; Grotius considers a wide variety of circumstances under which these rights of war attach and when they do not.
Book III takes up the question of what rules govern the conduct of war once it has begun; influentially, Grotius argued that all parties to war are bound by such rules, whether their cause is just or not.
The arguments of this work constitute a theory of just war. Roughly, the second book takes up questions of jus ad bellum (justice in the resort to war) and the third, questions of jus in bello (justice in the conduct of war). The way that Grotius conceived of these matters had, together with Francisco de Vitoria's De potestate civili, a profound influence on the tradition after him and on the later formulation of international law.
Britain's war on Denmark was illegal (and so was everyone else's war on a minor in this game--including my invasion of Piedmont).
