In September 1939, Britain (and France) did reluctantly go to war against Germany when it invaded Poland --- but hardly to protect this smaller, weaker nation from being being swallowed up by a bigger neighbour.
They basically sat on their hands while Germany, Slovakia and Russia invaded it and then divided it up.
Right to the end of the war and beyond, Britain (and now America) secretly horse-traded away the freedom of small nations; yes including the soon to be liberated Poland.
Traded then away without ever consulting their governments, let alone their populations.
The spirit of charity to weaker strangers (regardless of their economic or political value to the charity-giver) certainly did not animate any government during WWII.
The clash of arms of WWII did indeed happen between nations and as such, has been examined past the point of exhaustion by too many authors.
Examined but without however providing an unifying explanation to account for WWII's global indifference towards the lives of weaker strangers displayed by all three sides - Axis, Allies and Neutrals.
So to examine WWII's clash of ideals (because this disregard for the lives of weaker strangers was never universal at the individual level) we need to look well before and after 1945 and get down to the level of individuals .
Down right to the micro level , to examine over time the changing and differing opinions within families and between spouses.
For why was it so that a majority in every nation on earth in those years was so self confident that their indifference to the fate of weak strangers was scientifically (and hence morally) justified ?
I think we need to seek the origins for the universally brutal nature of WWII in the 1920s' changing attitudes towards the idea of charity, as displayed first at the Columbia Presbyterian Hospital and hundreds of former 'charity' hospitals like it all over the world.
It has long been noted (and then ignored) that during the Era of Modernity the leading explanations for how reality worked were biological and even medical --- rather than theological, philosophical or based upon the hard sciences of physics and chemistry.
Social Darwinism is thus mis-named : it really is Medical Darwinism.
For it was the medical examinations conducted during national universal conscription selection processes that provided the data for so many peacetime, non-military, social decisions.
So medicine did not sit on the sidelines on WWII , merely patching up the wounded ---- then current medical value systems began WWII's clash of ideals and only a change in today's medical value systems can finally end it.....