Obama's speech at Selma

The President spoke on Saturday on the 50th anniversary of 'Bloody Sunday'

President Obama spoke before thousans on Saturday during a commemorative ceremony for the 50th anniversary of the events of “Bloody Sunday” when over 600 non-violent protesters were attacked by Alabama state troopers as they attempted to march from Selma to Montgomery for voting rights (you can read subtitles)

Obama tells Selma crowd: 'Our march is not yet finished'

             

                               

Why were those events in Selma in 1965 called "Bloody Sunday "?
Why does President Obama  define the USA as "unique"?