Chapter 9- Atticus tells Scout that they had lost the case of Tom Robinson 100 years before it even happened. What is he referring to? Why would he still choose to defend Robinson if he knows he's going to lose?

Atticus knows that the racism is so entrenched in the South that a fair trial for Tom Robinson is difficult at best. The all white jury will undoubtedly be racist and Tom will lose. Atticus knows, however, that someone has to try for Tom Robinson. Nothing changes when old ideas are not challenged. Defending Tom is also the right thing to do and Atticus says he can never tell his kids what to do again if he does not take this challenge.