As someone that served in the military and in Iraq twice, there is a clear distinction between combatants and non-combatants.
In regular, non-total wars - like Iraq - that's true. A total war is defined by the mobilization of the entire population into the war effort, and this reduces the distinction between soldier and civilian. World War II was history's definitive total war. The Japanese - and American, and British, and German, etc. - populations were not doing anything
other than contributing to the war efforts, at the time.
My opinion is that people who join the military join willing to sacrifice their lives in war.
Both the Japanese and American militaries at the time were composed almost entirely of conscripts. Those people joined because their government ordered them to do so, under threat of imprisonment. There was no question of "willing." This was before the modern era of all-volunteer armies.
That is the precise distinction between us and civilians.
The distinction between soldiers and civilians is that soldiers participate in the war effort and civilians do not.
Or: the fact of conscription - that very few people on either side "chose" to be soldiers, or not - directly undercuts your position, there. You are arguing exactly that there was no distinction between soldiers and civilians in the context of WWII, because the exact distinction between them - choice to risk one's life by serving in the military - was systematically denied to the population.
There is no need to harm any civilians (people who are not participating in the threat against you).
There is nobody on the enemy side who is "not participating in the threat against you" in a total war. That's what makes it a total war, and legitimates the large-scale targetting of civilian infrastructure and populations. When that is not the case - in a traditional war - then, sure, you're exactly correct.
But let those who choose to fight fight and do not harm those who do not.
Since that choice was not left up to the individuals - it was made for them, by the governments, through the conscription process - I don't see the relevance of that suggestion.
We've moved on from the days of destroying entire populations of innocent lives in order to coerce their military.
It's done in order to coerce the political leadership.
And a big part of the reason that we have moved on from such, is exactly that people saw what happens when you go in for a total war against America (America hammers your country to bits from the relative safety of the far side of the world). So now total wars tend to get avoided. Nuclear weapons further increased the stakes in that regard.