The other force is one of bitterness and hatred, and it comes perilously close to advocating violence. it is expressed in the various black nationalist groups that are springing up across the nation.

why does the author include these sentences?
to predict the future of his group’s actions
to recall the harsh actions of law enforcement
to encourage understanding of black nationalists
to provide a contrast to his group’s nonviolent efforts

Well, since I had a look on the first words in the first sentence it become clear for me that the author included these sentence in order to provide a contrast to his group’s nonviolent efforts. Just because of the grammar structure of the lines, when it starts from ''The other'' we can predict that the speaker compares something with the excerpt represented above. I bet there was a previous introductory sentence that is connected  with particular movement and further we can see ''the other" that reflects contrast.