Why was the spirited young girl sent to live with a foster family in WWII Germany ?

Narrated by Death, The Book Thief is the story of Liesel Meminger, a nine-year-old German girl who given up by her mother to live with Hans and Rosa Hubermann in the small town of Molching in 1939, shortly before World War II.  Liesel's father was persecuted for being a Communist, and her mother was likely killed by the Nazis for the same crime.