No morals are not in the eye of the beholder. And I couldn't care less what the British thought.
Where are the rules for morality of warfare, to be found?
--Women, children, sneak attacks, infiltration, deception, weaponry bigger, bigger still.
--Treaties broken, alliances shifted, prisoner torture, and the list goes on and on.
Early writings can be found from the one Adomnan, Bishop of Iona.
"Cain Adomnain[edit]
Main article: Cáin Adomnáin
In 697, it is generally believed that Adomnán promulgated the Cáin Adomnáin, meaning literally the "Canons" or "Law of Adomnán". The Cáin Adomnáin was promulgated amongst a gathering of Irish, Dal Ríatan and Pictish notables at the Synod of Birr. It is a set of laws designed, among other things, to guarantee the safety and immunity of various types of non-combatants in warfare. For this reason it is also known as the "Law of Innocents"."