The pontiff said more should have been done by allied nations given the intelligence that showed people were being taken to death camps during the Second World War.
Francis also condemned the killings of Christians in Soviet Union death camps following the war.
He made the comments whilst speaking to young people on a visit to Turin in Italy.
"The great powers had photographs of the railway routes that the trains took to the concentration camps, like Auschwitz, to kill the Jews, and also the Christians, and also the Roma, also the homosexuals," he said, "Tell me, why didn't they bomb" those railway routes?"
Speaking about death camps under Stalin and the Soviet Union, Francis questioned: "How many Christians suffered, were killed?"
The Holy Father later spoke of the "great tragedy of Armenia" where many thousands of Armenians were killed by Turkish Ottomans.
He said: "So many died. I don't know the figure, more than a million, certainly. But where were the great powers then? They were looking the other way,"
Today's current conflicts around the world are like "a Third World War in segments", he said.