Even as Lemkin was formulating his concept of genocide, intentional group destruction on an unprecedented scale was taking place in his homeland, Poland, and members of his own family were victims.  

When the Nazi Party came to power in Germany in 1933, it immediately instituted an anti-Semitic campaign, an expression of its racist ideology, which asserted that Germans were members of a "master race" and Jews, Blacks, Roma, Sinti, and Slavs were "subhuman." German Jews were stripped of their citizenship and other civil rights.

During WWII, as Jews in other countries fell under German occupation, they too, were persecuted and isolated from their fellow citizens. They were imprisoned in ghettos and camps, where they were subject to forced labor, random killings, starvation, and disease.

In 1941, when Germany broke the Molotov-Ribbentrop Pact with the Soviet Union and invaded eastern Poland, the Baltic countries, Ukraine, and Russia,  the Nazis inaugurated a more systematic program of mass murder, deploying special killing squads to shoot down entire Jewish communities.

But it was in January 1942 at the so-called Wannsee Conference that the Nazis formally enunciated a plan for genocide, "The Final Solution to the Jewish Problem," whose aim was the total extermination of Jews.

The Nazi state mobilized every branch of government to participate in intentional group annihilation. Genocide was carried out on an industrial scale.Jews from the ghettos in Poland and from other German-occupied territories in Western and Central Europe, as well as another targeted group, the Roma and Sinti, were shipped by rail to concentration camps in Poland, including 6 killing centers: Chelmo, Treblinka, Sobibor, Belzec, Maidanek, and Auschwitz-Birkenau. These were facilities established primarily and exclusively for the assembly-line style mass murder of human beings. In these killing centers, millions of men, women, and children were murdered upon arrival or soon died from starvation, torture, or disease.

An estimated 6 million Jews were killed in the Nazi genocide. Although Lemkin, and his brother Elias and his wife and two sons survived the Holocaust (as it came to be called), the brothers lost 49 other members of their family, including their parents.

When the war was over,  Lemkin served as an advisor to the U.S. Supreme Court Justice and Nuremberg Trial Judge, Robert Jackson. He fought to have the word genocide introduced into the trial record, but his efforts were unsuccessful.