How did World War II become so all-encompassing so that the lines between combatants and civilians was largely erased?
The major reason for this was the development of technology. This led to total war in two ways.
First, it meant that a country's ability to win the war depended largely on its manufacturing. War was won by producing more tanks and bombs and airplanes, not by brilliant tactics. This meant that destroying enemies' manufacturing capabilities was important to winning the war. That meant that civilians in and around the factories would be targeted.
Second, it meant that there were huge bombers with massive bomb-loads but that those bombs would be inaccurate. Therefore, each side could attack the other's manufacturing and transportation centers but could not do so with any real accuracy. This meant that even civilians who were not targeted were very much in danger.