He also decried the deaths of Christians in gulags in Russia under the Stalin dictatorship, which followed the war.
The pope's harsh assessments came in impromptu remarks during his visit to Turin, northern Italy, when he told young people he understands how they find it hard to trust the world.
"The great powers had photographs of the railway routes that the trains took to Auschwitz to kill Jews, Christians, homosexuals, everybody," Francis said, citing the death camp in Poland, and asked: "Why didn't they bomb" those railroad routes?
Referring to the gulags in Russia, Francis said: "How many Christians suffered, were killed" there.
Lamenting the cynicism of world players in the 1930s and 1940s, Francis said: "the great powers divided up Europe like a cake."
He also cited what he called the "great tragedy of Armenia."
"In the last century, so many, millions, (of Armenians) died. But where were the great powers then? They were looking the other way," the pope said.
In April, the pope angered Turkey when he referred to the slaughter of Armenians by Turkish Ottomans as "genocide."
In today's world, he told the young people: "Everything is done for money." He criticized those advocating peace while manufacturing or selling arms.
Francis reiterated his view that conflicts in the world today are tantamount to "a Third World War in segments."