When Lincoln became president, he did not swear to end slavery, but instead just said he did not want it to spread beyond the existing slave states. Then the South left the union and the civil war began. Some people contend that Lincoln only freed the slaves as a strategy to help the North win the war, figuring it would be extremely difficult for the agricultural south to wage a war and address the loss of free slave labor at the same time.
After the Civil War, a group of southern white men started the organization known as the Ku Klux Klan. The primary ideology of the Klan was white supremacy: whites are inherently superior, blacks are inferior, white men must "control" the black population by all means available, including violence. The Klan provided leadership in a reign of violence, and they terrorized blacks, routinely lynching and murdering black men to re-gain control of the black population in the south. http://en.wikipedia.org/wiki/Ku_Klux_Klan
White crowds in the south often gathered to cheer on the Klan as a group of white men would storm into the home of a black man, beat him, kick him, drag him, without trial or justice, without any protection of the police, take the man out to a public area, surrounded by crowds of white American citizens, cheering, and then the white men would hang the black man from a tree, often lighting him on fire at the same time, while the crowds cheered.
The laws, and law enforcement, refused to even acknowledge the concept of the "murder" of a black person by a white person. This is the same demeaning attitude towards an entire group of people that led the law to declare that it was impossible for a man to rape his wife, and any woman out in public after dark by herself was probably "asking for it," and therefore wasn't "really" raped. Some groups of people in this country have always been considered lesser, and therefore the popular victims of violence.
http://withoutsanctuary.org/main.html (Photo source)
For black lynchings, there is actually a surprising amount of photographic evidence because taking pictures at lynchings was considered fun, and those photos were commonly reproduced and sold as postcards for people to send to their friends and family far away: wish you were here written on the back, the picture of a mutilated hanging dead black man on the front.
The Klan first began after the Civil War and has gained and lost membership and popularity at different times. From 1915 through the 1920s, the Klan became very popular and actively violent again, partly in response to the number of immigrants coming to the U.S. from Europe. During that period, it is estimated there were 4-5 million Klan members in the U.S. http://en.wikipedia.org/wiki/Ku_Klux_Klan
Today there are still Klan members in the country, but their numbers are in the thousands only. The real successors to the Klan seem to be right-wing political organizations and some religions that preach hatred of others and justify the use of violence, if not invite "god's violence" to be brought down against everyone who is different from the particular group.
When considering the life and the murder of Martin Luther King, Jr., it is imperative to view it in light of the centuries of violence in the U.S. against black men. No black person in the U.S. lives without a thorough understanding of the pervasiveness of white violence against blacks, and of their own family members going back generations who have been lynched, beaten, murdered by mobs of white men.
When black people in the U.S. decided to begin pressing for a betterment to their own status in society, to ask for a fair application of the laws, each and every one of them knew that the mere act of asking for fairness would subject them to possible murder. When Martin Luther King, Jr., began organizing and leading black Americans to ask the U.S. government to enforce their constitutional rights to equal protections of the law, equal treatment in our society, he used the tactic of nonviolence, and encouraged all his followers to be nonviolent in everything they did. Never raise a hand to a white man. Don't argue, don't fight back. Martin Luther King, Jr. knew that if black people stood up and defended themselves against attack, they would be slaughtered.
Yet even though he preached only nonviolence and lawful means of asking for an equal opportunity in our society, Martin Luther King, Jr. was murdered anyway. The simple act of asking for fairness received the death sentence.
Billy Holiday wrote the song "Strange Fruit" about the lynchings of black men in the South.