The Irish Republican Brotherhood was the earliest modern day terrorist organization. It was founded and organised by John O'Mahony and Michael Doheny.
The Irish Republican Brotherhood (IRB) was an organisation dedicated as a secret oath-bound fraternal organisation, dedicated to the establishment of an "independent democratic republic" in Ireland between 1858 and 1924. The organization seeking the establishment of a republic, the end of British rule in Northern Ireland, and the reunification of Ireland.
A lot of shooting and bombing attacks, killings was done by the organization.
The Ku Klux Klan was the world's first modern-day organized terrorist group. The Ku Klux Klan began in 1865 in Pulaski, Tennessee. It was classified as a white supremacy group or hate group. The KKK openly supported and funded the democratic party primarily due to their defeat in the civil war.
The KKK committed more bombings and had more deaths resulting from these bombings than any other terrorist group in history. They posed a serious threat to civil liberties. The KKK was a severe threat and did significant damage with unspeakable brutality against Blacks, Jews, and Catholics.