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One reason for a general decline in violence worldwide since World War II is that many nations and people no longer rely on preemptive violence, or attacking foes in anticipation of being attacked. The world is now bound closer by rules, trade, stable governments, and a greater knowledge of each other.

Yet this restraint against preemptive strikes is not assured. Just consider a few recent developments:

After North Korea fired a ballistic missile over Japan on Aug. 29, the Trump administration further raised the possibility of military action on the North’s nuclear facilities. “All options are on the table,” President Trump said. Even in pacifist Japan, nearly a third of people would favor a preemptive move.

Mr. Trump’s predecessor, Barack Obama, was ready in 2013 to launch a preemptive strike against Syria to prevent further use of its chemical weapons on its citizens. In 2003, President George W. Bush invaded Iraq on the premise that Saddam Hussein had the capacity to build or export weapons of mass destruction. And since the 9/11 attacks, all presidents have relied on “target killings” of terrorists deemed to be plotting attacks on Americans or others.

Closer to home, many members of extremist groups have justified violence during protests. On Aug. 12, a neo-Nazi demonstrator rammed a car into a group of anti-racist protesters in Charlottesville, Va., killing one. And in skirmishes in Berkeley, Calif., on Aug. 27, members of an anti-fascist movement (known as “antifa”) attacked right-wing demonstrators. Dartmouth College went so far as to issue a statement distancing the institution from the views of one of its lecturers, an expert on antifa, who supported violence by the “revolutionary left” movement against fascists during public protests.

The idea of preemptive attacks has a strong legal basis in international law. Nations are entitled to preemptive self-defense if an attack is considered “imminent.” But today’s threats are less clear-cut. Rogue states like North Korea or terrorist groups like Al Qaeda do not easily make known their intent or the exact whereabouts of their threat. And extremist groups in the United States rely on individuals or small groups to initiate attacks.

The world must be careful, however, not to easily accept preemptive violence as it once did. The days when kings, tribes, or clans would strike an enemy for the slightest threat have been replaced by a rise in organized governments granted a monopoly over the use of violence. And nations have steadily devised rules aimed at decreasing threats or they rely on international bodies, such as the United Nations, to justify a collective preemption attack.

As Harvard University professor Steven Pinker notes in his 2011 book, “The Better Angels of Our Nature: Why Violence Has Declined,” humanity has grown in its understanding and use of empathy as a tool for peace. Diplomacy and negotiation are preventing wars. The more people know of each other, the more they see their own good in the good of others. People’s “moral circles” have expanded.

The global trend away from violence, whether it be the murder of individual or a government’s use of armed missiles, requires vigilance to maintain it. And one of the best ways is to keep questioning those who too easily justify preemptive violence.