The Holocaust

The opposite of love is not hate, it's indifference.

The opposite of art is not ugliness, it's indifference.

The opposite of faith is not heresy, it's indifference.

And the opposite of life is not death, it's indifference. 
Elie Wiesel (1928-2016)

One of the things that made World War II different from other wars was that Nazi Germany was committed to goals that would lead to mass murder. The Nazi dictator Adolf Hitler had always had three goals. One was to destroy his opponents in Germany. A second was to make Germany the strongest country in Europe and to conquer Lebensraum, which means "room to live." This word implied that without this land, Germany could not survive: it was supposedly too small for its population. The third goal was to "purify" Germany—and then Europe—of "racial enemies" and to establish Germans as the "master race." These three goals were closely connected in Hitler's mind, and all three were mixed up with his hatred of Jews, which is known as anti-Semitism.

World War II Reference Library. Ed. Barbara C. Bigelow, George Feldman, Christine Slovey, and Kelly King Howes. Vol. 1: Almanac. Detroit: UXL, 2000. p. 157-179.