One common feature of a functioning nation state is a social contract
that delegates all legal use of force to the government of that particular state. This is called the monopoly on violence
and implies that only the state and its institutions can claim to rightfully use physical force.
Needless to say that there are many instances in which this monopoly is called into question by sub-state actors. Such groups are given varying titles. In the case of the Islamic State
for example, most observers simply refer to it as a terrorist
organisation. There are other groups, that employ violent means for political ends whose labelling is more contentious, for example because they oppose an illigitimate or authoritarian state.
Calling a group terrorist therefore depends on which side the person labelling the organisation stems from. This is why many scholars avoid the term terrorism alltogehter, even though its use has become inflationary ever since the 9/11 attacks and the start of the so-called war on terror
15 years ago. Unlike the positive term freedom fighter, or the more neutral term rebel fighter, the label terrorist is always used in a negative way.
Based on research by the IHS Jane’s Terrorism and Insurgency Centre
the below chart gives an idea of which groups are the deadliest and where they're predominantly fighting. Please keep in mind that this does not mean that these groups are associated with the state they fight in.