God is a peaceful God because in Him there is an eternal order, harmony and peace, that is, there is a loving relationship between Father, Son and Spirit which is the very essence of God. God does not exist without order, harmony and peace . His commandment is, "Do not kill" because mankind is meant to participate in the peace of God, but killing is justified in self-defense or in the defense of the innocent.
This being said, the annals of war are replete with inhuman acts toward man - rape, abduction, torture, mutilation, starvation, murder and slaughter of innocent people. In moral theology there is a principle which states that "The end does not justify the means." For example, torture is not a just mean of obtaining information. Likewise, Hitler extermination of the Jews in the Second World War was not justified regardless of whatever good would come because of it. The indiscriminate killing by the US dropping of the atomic bomb over Hiroshima and Nagasaki in order to shorten the war and spare American lives was not justified. All of these acts were done under the presumption that the good end justifies whatever means are used to achieve it. An immoral act cannot be made just by the good intention or the good it seeks to achieve. Today terrorist groups work under the same principle that "the end justifies the means". They use suicide as a means of killing innocent people and what is even worse they justify it as "Holy War" as if they were carrying out the will of God.
Most people have heard of the Geneva Conventions which established rules of conduct in war but few of us know how it came about.
On June 24, 1859 the Frech-Piedmontese troops engaged the Austrians in Northern Italy near the towns of Solferino and San Martino. It involved 300,000 soldiers over an area of 12 1/2 miles of broken country.
Henri Durant, a visiting Switz, was an eye witness and here I quote part the savagery he saw, "Here is a hand-to-hand struggle in all its horror and frightfulness; Austrians and Allies trampling each other under foot, killing one another on piles of bleeding corpses, felling their enemies with their rifle butts, crushing skulls, ripping bellies open with sabre and bayonet. No quarter is given; it is a sheer butchery; a struggle between savage beasts, addened with blood and fury. Even the wounded fight to the last gasp. When they have no weapon left, they seize their enemies by the throat and tear them with their teeth." "From the midst of all this fighting, which went on and on all over the battlefield, arose the oaths and curses of men of all the different nations engaged -men, of whom many had been made into murderers at the age of twenty!" (Henri Durant, A Memory of Solferino, Published by the International Committee of the Red Cross, P.5 & 8) "When the sun came up on the twenty-fifth, it disclosed the most dreadful sights imaginable. Bodies of men and horses covered the battlefield; corpses were strewn over roads, ditches, ravines, thickets and fields; the approaches of Solferino were literally thick with dead." (Ibid., p. 11)
When the carnage was over there were 40,000 wounded. "Those wounded who were able to do so headed for the nearest village -- Castiglione -- in search of a little food and water; 9,000 reached it, pouring into houses and barns, squares and narrow streets. In the church of Castiglione, the Chiesa Maggiore, Henri Dunant, helped by local women, cared for the wounded and dying for three days and three nights." (Webpage: The Battle of Solferino /24 June 1859)
This experience left a deep impression on Henri Durant. In 1863 he wrote a book describing the horrors of the battle and proposed that every nation should create a voluntary relief society to care for the wounded in war and that field hospitals and personnel should be declared neutrals in conflicts. The emblem chosen for these organizations was the Red Cross against a white background. In 1864 twelve nations at Geneva ratified the "International Humanitarian Law". This became known as the First Geneva Convention.
"In 1901, Dunant was awarded the first-ever Nobel Peace Prize for his role in founding the International Red Cross Movement and initiating the Geneva Convention."