How did hilter and the nazis kill 6,000,000 jews without anyone stopping them?

While, it was more of a slow process at first,
In the beginning years of the war, Hitler's Priority was to round up all the Jews
He would use them for slave labor to build his war machine
and once they could no longer work, he would kill them.
Because it was mainly the S.S. doing everything, no one could or would defect to tell anyone
Also O.S.S. or the French had not seen what the Germans were doing because then they would have to sneak into the camp and not be killed.
So around the middle of the war, they decided (because they started to lose) that they were going to start killing all the Jews. 
So through mass murder they started killing everyone, and because no one knew about it till weeks before the Nazi's surrendered, no one had tried to stop it.
Also the camps were normally in the middle of nowhere which made things easier.