In the Pacific theatre, the Allies waged war with tremendous ferocity. Initially, they believed that the Japanese were myopic weeds with big glasses and buck-teeth, evidently being unfamiliar with atrocities in the Sino-Japanese war which may have killed up to 15 million. 

Allied troops rapidly revised their opinion as Yamashita's troops ran through them in Malaya, and forced a much larger Dominion force to surrender Singapore under humiliating circumstances. They discovered that the Japanese attitude to prisoners of war had evidently undergone a sea change since the 1904-5 Russo-Japanese War, in which the enemy had been treated with conspicuous decency. 

It may be that the intervening racial war the Japanese had fought in China lowered respect for Geneva Conventions the Japanese had signed "with amendments" but without ratification. Their own surrender was not prohibited, but it was highly stigmatised, not least by the audience of august ancestors watching the soon-to-be-dead. A projection of how Japanese imagined surrender themselves, combined with a need to advertise humiliating white men while putting other Asians in their "proper place", may explain the appalling treatment the Japanese inflicted on their captives. Malevolent sadism, often by ethnic Koreans as well as Japanese, had its own grisly momentum. 

Allied troops waged their own war of extermination against the Japanese, with slogans such as "Rodent Exterminator" stencilled on their helmets. This was especially after they encountered (or heard about) cannibalism, torture, mutilation and faked surrenders, though sheer racism towards an incomprehensible enemy played its part. 

Since both sides knew what was going to happen to anyone seeking to surrender, the most prudent calculation was to fight to avoid fates worse than death. This translated in the Japanese case to men blowing themselves apart with hand grenades or being mown down in massed banzai charges. The normative view among their opponents was succinctly put by the US Marine sergeant who said: "We'll have to kill every little yellow bastard." In a reversion to practices used against the Apache, or during the US conquest and occupation of the Philippines earlier in the century, trophy hunting became commonplace, with US troops collecting bags of enemy ears and wearing bracelets of Japanese teeth. Critically wounded Japanese prisoners were not safe either, as George MacDonald Fraser noted when Indian troops surreptitiously buried them alive under rocks in a Burma field hospital. 

So far I've offered what you may like to think of as a moral map, rather than a compass. The implicit lessons are banal. Don't elect governments which practise predation against other nations. Invest financial, moral and intellectual capital in defusing conflicts, before they accelerate into fighting. Fight existential wars implacably, but avoid those which are not of necessity. And accept that since wars are going to be around for a long time, we need to be prepared to wage them, while seeking to do so in a just and proportionate fashion, with officers in firm control of men who might be tempted to stray on to the dark side.