(CNN) - It’s easy to wonder why more people didn’t see the Holocaust coming.
Albert Einstein did and some of his letters discussing it are up for sale.
Three letters he wrote showing how he perceived Hitler and the rising wave of anti-Semitism in Germany…are going up for auction.
In a 1921 letter addressed to his sister, Einstein, a German born Jewish physicist, declined a trip to Munich because it was not safe.
In another letter dated 1934, Einstein sent money to his first wife to care for their schizophrenic son.
"All this is the result of the Hitler-insanity, which has completely ruined the lives of all those around me ..." Einstein wrote, according to the auction house.
The letter was written the year after Einstein moved to the U.S. and renounced his German citizenship.
Bidding for the letters starts Thursday, with one of them starting at $25,000.
Einstein also wrote a letter to President Franklin Roosevelt in 1939 urging him to build an atomic bomb.
The bomb he advocated for ultimately helped end WWII.
That letter is not up for auction.
CNN reports, an estimated six million Jews died at the hands of the Nazis and their allies in the Holocaust.