World war two started when Germany invaded Poland on September 1, 1939. Two days later, this brought France and Great Britain into the conflict. On September 17, the Soviet Union invaded eastern Poland but no reaction was provoked. By the spring of 1941, Nazi Germany had conquered or dominated all of Europe except Switzerland, Sweden, and Vatican city.
Nazi Germany and its allies killed six million Jews, along with other persecuted groups such as the physically and mentally disabled, Jehovah's witnesses, dark-skinned people, and gypsies. Holocaust victims, mainly Jews, were taken to concentration camps, where they were given forced labor and eventually died. Some of them died from starvation and others were killed by the Nazis.
Out of all the six million Jews killed, 1.5 million of them were children, along with the other non-Jewish children. Some children were forced to participate in medical research experiments. The experiments resulted in death, trauma, disfiguration, or permanent disabilities. The children were used to test immunizations, various poisons, and surgical procedures without anesthesia.