WWII was called "the good war" because the US was fighting for its very survival. The Axis Powers were a strong threat to the US's economic and diplomatic allies; in time, many thought that Germany and Japan would directly threaten the US as well. War correspondents from Britain and China portrayed the Axis military forces as inhumane for their willingness to attack civilian targets. Hitler, Mussolini, and Tojo were also leaders who were bent on world domination; the US saw itself as the good guy in the fight of good versus evil.
The objectives for this war were clearly stated at the onset. The goal was to roll back Axis gains and force the Axis powers to surrender unconditionally. There would be no separate peace or cessation of hostilities until this goal was met. The Japanese attack on Pearl Harbor incensed many Americans into believing that war was the only response to these belligerent nations. In fighting these powers, there were defined military objectives, such as the attack of armies and the bombing of industrial centers.
While there would be arguments of waging the war for humanitarian reasons, this was not the most important goal of the war. The US had its own racial issues and it partnered with the Soviet Union to fight the Nazis. The Soviet Union was notorious for its treatment of dissidents and minorities. The US was fighting against the fascist takeover of the world; the humanitarian reasons would be added later during the war crimes trials in Germany and Japan.