The concept of the limited, professional army type of war went out the window in the late 1800s. Up until then you basically fielded a professional army numbering in the tens of thousands. It took a lot of work to field that army. The army was equipped with expensive and difficult to (hand) produce weapons. It was supplied as best you could for a single extended campaign. There would be some limited resupply but not much, with living off the land accounting for much of the rest.
You then marched your army to war. It fought a handful of battles, perhaps one major one, that pretty much decided the war. Then the two cousins -- prince fluffy shirt and duke dandy pants -- negotiated a peace giving up a province or two and that was that. Civilians were largely unmolested as a rule (well, at least in more modern times), or at worst had their food stocks depleted when they crossed paths with an army.
Then, you had the arrival of the industrial age with mass production, nationalism and conscription. The US Civil War was a tipping point. Suddenly, you can crank out large numbers of troops (using conscription and mass training techniques) move them to the front using steam power (railroads and ships), supply them on campaigns using steam power, crank out munitions in absurd quantities and fight devastating wars over many years where millions die in a constantly refilled meat grinder until one side reaches total exhaustion and the war ends.
With an industrial war there are no longer civilians. Civilians are absolutely essential for keeping the ranks filled with men, and the supply lines filled with bread and bullets (and planes, torpedoes, centmetric radar valves, ball bearings, food, armor plate, canvas ammo pouches, helmets, tanks, optical sights, pressure bandages, shoe laces, machine guns, machine gun tripods... etc) Even someone who drives a civilian bus supports the war effort transporting troops and workers behind the lines. The truly innocent do exist, the children and pacifists, but by and large modern history suggests that the civilians buy into the war and support it from the start to even a bad finish.
Now, when you have millions dying on the battlefield instead of tens of thousands it becomes a lot easier to justify targeting enemy civilians if it promises to save the lives of your soldiers. Break their will to fight and end the war. Kill them and leave the factories empty. Destroy the factories themselves. Blockade the island and starve them to the peace table…whatever. Shorten the war a year and you save hundreds of thousand or even millions of your citizens lives. If you are the non aggressor, the difference between a soldier (drafted to fight a defensive war) and the civilian population of an aggressor become very blurred indeed.
A modern industrial war simply cannot function without the country's civilian population and supply and production infrastructure. No less valuable than the divisions and fleets doing the fighting.
So, IMO in a total industrial war targeting civilians – OK. Shooting pilots in parachutes over their home territory – OK. Machine gunning the Japanese troops in the water of the Bismarck Sea – OK (many actually did make landfall in the combat zone). Attacking military support personnel – obviously OK.
But all is not lost! The key for civilians is to be very conservative about the people you elect and support since if you make the wrong choices, your nights may be filled with the drone of Merlin engines and the pop of incendiaries. As exciting as the fall of France and Poland (to name a few) might have been, you better hope the boys in black in Berlin have their stuff together for the big finish. You can always take to the streets and end the war yourself.
The nuclear age of course, changes this with MAD. And, you have to wonder if we have embraced technology to the point that a total conventional industrial war is even doable today. Once you burn through your jets and missiles and high tech MBTs and subs the replacement rate will have to be pretty slow. Perhaps you then revert to 1916 on the Western front.
In 4th generation warfare like we have had in Vietnam and Iraq, targeting civilian populations not only fails to serve a military function but has a negative impact on achieving a positive outcome. There is no war production to speak of, except for the manufacturing of insurgents and insurgent supporters and bombing and abusing civilians only facilitates that process.
Charon