The horrendous events of World War 2 were, of course, much on Lewis's mind as he wrote Mere Christianity. Here he is referencing Germany's conquering of Poland, which began on September 1, 1939. The cost of the war in terms of human suffering in Poland is almost incomprehensible.
According to www.kasprzyk.demon.co.uk, "Over half a million fighting men and women, and 6 million civilians (or 22% of the total population) died. About 50% of these were Polish Christians and 50% were Polish Jews. Approximately 5,384,000, or 89.9% of Polish war losses (Jews and Gentiles) were the victims of prisons, death camps, raids, executions, annihilation of ghettos, epidemics, starvation, excessive work and ill treatment. So many Poles were sent to concentration camps that virtually every family had someone close to them who had been tortured or murdered there."
And, from wikipedia, "In 1942, the Germans began the systematic killing of the Jews.... Six extermination camps (Auschwitz, Belzec, Chełmno, Majdanek, Sobibór and Treblinka) were established [in Poland] in which the most extreme measure of the Holocaust, the mass murder of millions of Jews from Poland and other countries, was carried out between 1942 and 1944. Of Poland's prewar Jewish population of 3 million, only about 369,000 survived the war."