Scholars have also attempted to organize terrorist groups by historical “waves.”
The European anarchists of the late 1800s, considered the first modern terrorists, are usually designated as the first wave.
The anti-colonial, ethnonationalist wave began in the 1920s, coinciding with the surging popularity of self-determination after World War I.
The New Left wave, beginning in the 1960s, was largely a response to the Vietnam War. In the United States, it manifested in left-wing groups such as the Weather Underground.
Since the 1980s and particularly since 9/11, religious groups have been the dominant terrorist entities around the world.
In the United States, right-wing extremists are responsible for the greatest number of violent attacks. Longstanding U.S. right-wing groups re-emerge periodically, often in response to social changes they dislike. The Ku Klux Klan, for instance, which got its start in the 1860s as a paramilitary force repressing black emancipation, was popular in the 1920s when it opposed Jewish and Catholic immigration and in the 1960s during the civil rights movement.