What were two root causes for groups to turn to terrorism in the twentieth century?

During the 20th century, various groups used terrorism as a tool for advancing their ideologies and agendas. Such groups included nationalist movements like those seen in Nazi Germany and the Soviet Union, these early police states were unknowingly using many practices associated with terrorism including massive police surveillance systems, fear and informing networks, and the use of torture. Technological advancements of the 20th century also made terrorism a more attractive tactic for revolutionary or anarchist entities, the threat nuclear weapons and the use of chemical weapons have proven effective tactics of terrorist groups. 

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.