During WW2 our understanding of a moral war was not a war in which we did not kill any civilians (we killed a lot of civilians), it wasn’t even a war in which we did not kill any civilians on purpose (we killed a lot of civilians on purpose), it was a war in which we did not kill civilians without having a good reason.
We might carry out mass bombings of entire cities to destroy the enemy’s wartime production capabilities and demoralize his population.
Until recently, those were considered good reasons for killing civilians.
The moral context for these actions, snipped away from anti-war works such as Slaughterhouse-Five or Grave of the Fireflies which reduce the American bombings of Dresden or Kobe to the senseless acts of brutal monsters, is that we were fighting Germany and Japan using their own tactics against them.
It was Germany which introduced the bombing of cities to Europe during WW1 and WW2. In 1917, after German bombings, Premier Lloyd George shouted to a working class London crowd, “We will give all back to them and we will give it to them soon. We shall bomb Germany with compound interest.”
WW2’s Blitz was repaid with compound interest over Germany. Japan’s firebombing of Chinese cities was repaid with compound interest with the firebombing of Tokyo. This wasn’t mere vengeance. The rules of war are set by mutual consent. The humanitarian protections that we have come to take for granted as if they were natural laws are really mutual agreements between two sides.
On September 14, 2001, George W. Bush stood at Ground Zero and offered the working class New Yorkers amid the rubble an echo of Lloyd George. “I can hear you, the rest of the world hears you, and the people who knocked these buildings down will hear all of us soon.”
Since then we have been mired in an extended debate which presumes wrongly that any laws of war ever applied to those men. The entire existence of terrorism is eloquent evidence that treating fighters who have placed themselves outside the social contract as if they were within it is foolishly destructive.