In 1937, Adolph Hitler's Luftwaffe indiscriminately bombed the Spanish town of Guernica, killing 1,650 civilians. The New York Times reported, "The object of the bombardment seemingly was demoralization of the civilian population....not a military objective." Soon after, America rightfully denounced the bombing as a "monstrous crime."
But this wasn't the first time anybody had bombed civilians. The French, Germans, and British had all partaken in limited civilian bombings in World War I — killing thousands.
In 1925, France and Spain defeated a Berber uprising in Morocco by use of civilian bombings. American volunteers, under French command, bombed the city of Chechaouen, similar in size to Guernica. And from 1926 through 1928, the US Marines utilized civilian bombing to force regime change in Nicaragua. There was no public outcry within the United States for either of these actions. For some reason, these were not considered crimes.
But when Japan bombed civilians in Shanghai in 1932 and claimed thousands of lives, the New York Times reported that those bombings brought a "literal avalanche of denunciation" upon Japan (and rightfully so!) 
In fact, it is said that the Shanghai bombing caused Americans "to view the Japanese as 'butchers' and 'murderers'." And again in 1937, when Japan again bombed Shanghai, the bombing was viewed correctly by Americans as "an atrocity of the most appalling kind."
In 1938, as a result of all of these civilian massacres (and others), the League of Nations unanimously passed a resolution outlawing "the intentional bombing of civilian populations." In 1939, nearing the outbreak of World War II, FDR made a public plea that the warring parties refrain from the "inhuman barbarism" of bombing civilian populations, acts which "sicken[ed] the hearts of every civilized man and woman," and "profoundly shock[ed] the conscience of humanity." As a result, Hitler pledged he would limit his air force to attacking only military targets. And British Prime Minister Neville Chamberlain stated that "Britain will never resort to the deliberate attack on women and children and other civilians for the purpose of mere terrorism."
Article comments
1 - Victor Plenty
When war is total, civilians are targets. There is nothing good about this, but it is the reality of modern warfare.
Civilians will not be safe until the war is over.
The reality of a world with nuclear weapons technology is simple and direct. Nobody will be safe until everybody is safe.