Let me explain in my own words rather than copy and paste.
After the defeat of the confederate south in the american civil war, a group of renegade confederate officers mainly of scottish descent founded the KKK to regain the honour the South had lost in the war.
Basically they were a bunch of hicks who went around terrorising black people, jews, catholics etc, it was then banned and dissappeared for the next 50 years.
Prior to the Great depression they returned and exploited the immigration situation to recruit young white males, this is when they were their strongest, this time they were anti-black, catholic, jew, eastern european, basically they contnued doing the same old crap, lynching blacks, beating up catholics burning cross's and playing dress up.
then world war 2 came along and people started to leave to fight the nazi's, since then it's been a dwindling organisation, with minor resugence's during the Martin Luther King period where they went around killing black kids, then the government cracked down on them and they started to dwindle again.
Since the post 1945 period white supremacists found themselves joining the numerous Neo-Nazi organisations rather than the KKK. Many didn't like the semitic origins of the Christian religon the KKK practiced.
read this and ye shall learn.