In 'To Kill a Mockingbird', what are the historical aspects of racism and why does Atticus feel such a need to take Tom's case?

In the 1930’s, Alabama was exposed to a great injustice in
the Scottsboro trials where nine African-American males where unfairly accused of raping
a white woman.  In spite of evidence which proved the men’s innocence, the men were
sentenced to death.  This outraged many human rights activists.  As a reaction to this
incident, Harper Lee, the author of To Kill a Mockingbirdwrote about a similar incident,
while conveying a strong message to society.  Her theme of prejudice is developed by the
characters of Boo Radley, Tom Robinson, and through the symbol of the mockingbird.  Lee
is obviously sending a strong message that this kind of bigoted attitude is unacceptable
and must be stopped.

This is also why Atticus feels
compelled to take on the Robinson case.  He feels that it is our duty as citizens to
bring change if it is possible.  Although he knew that defending Tom would bring him and
his family nothing but heartache; he knew it was the right thing to do.  He felt it
necessary to show his children, his society’s future, the proper example of empathy and
standing up for your beliefs, no matter how difficult that may be.  Consequently, this
also brings about a major theme in the novel, that of the true meaning of
courage.