Steven Pinker writes

the 17th-century philosopher Thomas Hobbes got it right. Life in a state of nature is nasty, brutish, and short–not because of a primal thirst for blood but because of the inescapable logic of anarchy. Any beings with a modicum of self-interest may be tempted to invade their neighbors and steal their resources. The resulting fear of attack will tempt the neighbors to strike first in preemptive self-defense, which will in turn tempt the first group to strike against them preemptively, and so on. This danger can be defused by a policy of deterrence–don’t strike first, retaliate if struck–but to guarantee its credibility, parties must avenge all insults and settle all scores, leading to cycles of bloody vendetta.

These tragedies can be averted by a state with a monopoly on violence. States can inflict disinterested penalties that eliminate the incentives for aggression, thereby defusing anxieties about preemptive attack and obviating the need to maintain a hair-trigger propensity for retaliation. Indeed, Manuel Eisner attributes the decline in European homicide to the transition from knightly warrior societies to the centralized governments of early modernity. And today, violence continues to fester in zones of anarchy, such as frontier regions, failed states, collapsed empires, and territories contested by mafias, gangs, and other dealers of contraband.

We certainly can point to state-inflicted violence in this century: Hitler’s Germany, Stalin’s Russia, and so on. However, Pinker points out that the rate of violence has actually declined over the course of history. He does not necessarily view the emergence of strong states as the cause for this decline.