Terrorism is a form of asymmetric warfare, and is more common when direct conventional warfare will not be effective because opposing forces vary greatly in power. Yuval Harari argues that the peacefulness of modern states makes them paradoxically more vulnerable to terrorism than pre-modern states. Harari argues that because modern states have committed themselves to reducing political violence to almost zero, terrorists can, by creating political violence, threaten the very foundations of the legitimacy of the modern state. This is in contrast to pre-modern states, where violence was a routine and recognised aspect of politics at all levels, making political violence unremarkable. Terrorism thus shocks the population of a modern state far more than a pre-modern one and consequently the state is forced to overreact in an excessive, costly and spectacular manner, which is often what the terrorists desire.