I came across a very good definition of the Laws of War in GF Martens, A COMPENDIUM OF THE LAW OF NATIONS FOUNDED ON THE TREATIES AND CUSTOMS OF THE MODERN NATIONS OF EUROPE ... translated by William Cobbett, London, 1802.

p. 286
"... war gives a nation an unlimited right of exercising violence, against its enemy. But, the civilized powers of Europe, animated by a desire of diminishing the horrors of war, now acknowledge certain violences which are as destructive to both parties as contrary to sound policy, as unlawful, though not entirely forbidden by the rigour of the law of nations. Hence those customs which are at present called the laws of war.

These laws, which are sanctioned by custom, and in some cases even by treaty, have been observed with greater punctuality since war has been carried on by the means of regular troops: nor does any civilized nation now think itself justifiable in departing from them, except the enemy sets the example, or except an urgent necessity, arising from extraordinay circumstances, admits of exceptions authorized by reasons of war."

DG