North Korea backed away modestly from its threat to surround Guam with a ring of fire on Monday, a surprising de-escalation in the war of words that seemed late last week to be building toward an actual armed conflict. The Wall Street Journalprovides a useful timeline of the crisis here.

In the accompanying story, the Journalnoted that Pyongyang made its announcement “hours after China took its toughest steps against Pyongyang to support U.N. sanctions,” including a pledge to “ban imports of North Korean coal, iron and seafood.”

It is possible that such economic pressure convinced the DPRK to rethink its approach. But a less-noticed Chinese statement might have had a bigger impact—both on Pyongyang, and hopefully here in Washington.

Last Friday, China’s Global Times explained that Beijing should not come to North Korea’s aid if the hermit kingdom launches missiles against the United States, but that China wouldhave North Korea’s back if it was the victim of U.S. or South Korean aggression.

As the Washington Post noted, the “comments reflect the 1961 Sino-North Korean Treaty of Friendship, Cooperation, and Mutual Assistance, which obliges China to intervene if North Korea is subject to unprovoked aggression—but not necessarily if Pyongyang starts a war.”

The Global Times’ editorial, believed to reflect official Chinese government policy, clarifies the frequently muddled difference between preemption and prevention. The former is a legitimate right of self-defense. The latter is functionally indistinguishable from aggression.

A country that has knowledge of a direct and imminent threat to its citizens is not obligated to wait until after the missiles fly or the bombs fall before taking action. If a country launches a war in order to prevent a future threat from materializing, however, such actions are likely to be roundly criticized—and, in this case, would activate the Sino-North Korean alliance. Make no mistake: China was issuing a deterrent threat to both the United States and North Korea.

The distinctions between preemption and prevention frequently become blurred, and are often deliberately misconstrued. In his speech at West Point in 2002, for example, President George W. Bush said that Americans had “to be ready for preemptive action when necessary to defend our liberty and to defend our lives,” but the war he launched in Iraq less than ten months later was a classic case of preventive war. Bush used military force to overthrow Saddam Hussein’s government, not because he had evidence of an imminent attack against the United States, but “to disarm Iraq, to free its people and to defend the world from grave danger.”

Of course, Bush’s father had attacked Iraq twelve years earlier, but he had launched that war to reverse Iraqi aggression against Kuwait. Not so surprisingly, the first Gulf War enjoyed broad international backing. Bush 43’s Iraq War, by contrast, engendered strong opposition abroad.

Mindful of the reaction that naked aggression is likely to evoke, many past leaders and rabble rousers have capitalized on minor incidents as a pretext for war (think, for example, of James K. Polk and Zachary Taylor in Texas, 1846; William Randolph Hearst and “ Remember the Maine” in 1898; and LBJ and the Tonkin Gulf in 1964).

Still others have manufactured bogus cases of aggression by others. In September 1931, Japanese soldiers planted explosives near a Japanese railway in Mukden (Shenyang), China, and then used that as a justification for launching a notoriously brutal war. Adolf Hitler asserted that Poland started the war against Nazi Germany in late August 1939, a claim so transparently untrue—based on a series of false flag operations involving Germans dressed up in Polish army uniforms—that it is often forgotten. However, the fact that some of history’s most ruthless aggressors would go to such elaborate ends to create the appearance of having acted preemptively, as opposed to preventively, demonstrates the importance of that distinction.

The psychology of preemption versus prevention is equally relevant in interpersonal disputes. When schoolyard scuffles or battles between siblings break out, “He/she started it!” is usually the first thing that a teacher or parent hears. It is the critical piece of evidence to adjudicate guilt or innocence. To defend oneself is noble; to attack others is treacherous.

Alas, there are likely to be numerous instances in which U.S. and North Korean forces could come into contact in the near future. Donald Trump could seize upon any one of them, initiate a wider conflict, and claim that he was acting in self-defense (i.e. preemptively), in order to rally the public here at home, and fend off international criticism.

But if Trump is determined to launch a preventive war, in order to secure a quick win, or boost his flagging popularity, or merely because he doesn’t like the young punk with the bad haircut, he should tread carefully. China has clearly stated that it won’t sit idly by if the United States is responsible for starting a new war in Asia.