Last month, during President Barack Obama’s recent barnstorm through East and Southeast Asia, he announced in a joint press conference with his Vietnamese counterpart Tran Dai Quang that the embargo on weapon sales to Vietnam was to be lifted. Though the White House had hitherto reassured human-rights watchers that any negation of this Cold War–era policy would be directly tethered to Hanoi’s record of improvement on issues of freedom of conscience (admittedly described by Obama as “modest”), what ended up proving more important in the eyes of Washington officialdom was what Harold Macmillan once described as the primary determining factor in politics: “Events, dear boy, events!”

For Obama, the event foremost in mind is the frightening potential for a hollowing-out of the ambitious Pivot to Asia he christened seven and a half years ago. While the president has allowed his foreign-policy focus to be distracted by the Middle Eastern maelstrom as well as a revanchist Russia, he is not entirely to blame; indeed, both the Democratic and Republican presidential nominees have done nothing to massage Pacific Rim interests. Despite much of the language of the Trans Pacific Partnership (TPP) having been crafted and negotiated in Secretary Hillary Clinton’s State Department, Candidate Clinton, sensing a rising gale coming in from her left, wrenched her campaign’s tiller sharply to port , tacking with the wind of Bernie Sanders’ “Revolution.” She survived the tempest, but her ability to swing back toward a pro-TPP position is effectively nil—to paraphrase yet another British prime minister, Winston Churchill, politicians can easily rat; it’s the re-ratting that comes far harder. And of course, Donald Trump’s rhetoric regarding the Japanese and South Korean alliances has caused its share of tremors .

For the Vietnamese, however, the “event” is not so much a single pinpoint in time as eons of accrued mistrust toward their northern neighbor. The first recorded Chinese invasion of Vietnam was back in the second century BC, when Emperor Qin Shi Huang expanded his newly united China into the reaches of northern Vietnam. This state of affairs, with the Chinese more or less exercising suzerainty over a Vietnamese client kingdom, lasted until 1884 when the French became the new colonial masters in Southeast Asia. It was ultimately Washington’s desire to buttress France’s status in Vietnam that in fact led to the first American involvement under President Truman . Yet all the while, Ho Chi Minh kept in mind who the more ominous foe was, actually working with the French to get the Chinese Nationalists out of northern Vietnam after World War II.

In 1979, ten years after Ho’s own life had come to an end, his suspicion was tested, with two hundred thousand soldiers of the People’s Liberation Army amassed on Vietnam’s northern border ready to invade. Their failure to make a substantive dent in the guerilla tactics that had served the Vietnamese so well against the French and Americans is an obvious takeaway. But in the larger realm of history, there are further lessons to be gleaned. Here are four others:

1. America’s involvement in Vietnam was founded on a faulty premise, and the Sino-Vietnamese War proved it.

Ever since President Eisenhower had employed the metaphor of dominos toppling one after another to explain the threat of Communism to the nations of Southeast Asia in the aftermath of Mao’s 1949 victory in the Chinese Civil War, this notion served as shorthand for the prevailing wisdom dictating American Cold War policy. The advance of Communism must be stopped in its tracks, the theory went, because the nations that had already turned red were in lock-step and had abandoned historical grievances in pursuit of the overriding common goal of spreading Marxist ideology. This was the logic that led Washington policymakers to defend South Vietnam for nearly two decades. But viewing the Communist threat as a monolith could have been prevented as early as the late 1950s when rumors of the Sino-Soviet split were starting to emerge (ironically, the only State Department veterans capable of analyzing such a development, the fabled China Hands, had seen their careers hammered by McCarthyist scare-mongering). In playing the 1972 opening to China off the pursuit of détente with Brezhnev’s Soviet Union, the Nixon/Kissinger duo signaled their appreciation for these intra-Communist fault lines.

It was the Sino-Vietnamese War that made these fault lines plain for all to see. In 1978, Vietnam, tiring of the instability caused on the Vietnamese/Cambodian border by the Khmer Rouge government in power since 1975, launched an offensive into Cambodia and took Phnom Penh. However, in the great game of Moscow/Beijing rivalry, this was interpreted (correctly) as a Moscow-aligned nation making war on a Beijing-allied nation. In a minuet somewhat reminiscent of August 1914, China, which could not allow this affront to its ally to go unanswered, intervened against Vietnam, staging the invasion that led to the Sino-Vietnamese War. It is interesting to note as well that before it was clear the Vietnamese would hold their own against the PLA, the Soviet Union was supplying Hanoi with materiel and had already dispatched naval support to the South China Sea to assist in intelligence-gathering.

2. Beware a new leader who needs to prove his strength. He might just lash outward.

By 1979, the diminutive Deng Xiaoping was newly ensconced in power, having seen off the rival threat posed by the ultra-Maoist Gang of Four (headed by Mao’s fourth wife, Jiang Qing). Deng intended for his leadership to be disruptive and a break from Maoist orthodoxy. However, the lurch toward economic liberalization that defined the Chinese experience after the 1980s could not have occurred without Deng first consolidating power and proving his capacity for leadership (the old imperial concept of the Mandate of Heaven, while no longer formalized, has never really exited the Chinese imagination). Unsurprisingly, waging war against a historic and long-standing enemy was the surest means to achieve both.

Additionally, for a Chinese leader to ascent to the summa of paramount leader, his power must rest on an effective tripod: control of the state, control of the Communist Party, and control of the military. In choosing to fight where—and more importantly, when—he did, Deng may have bought himself precious time in his first full year in charge to cement his own power in Beijing while the PLA was too distracted by an active campaign to throw up any hurdles.

3. The ethnic minority as a readily available excuse.

Whether it be Hitler’s insistence that Sudeten Germans were being marginalized in Czechoslovakia, Putin’s belief in Russia’s role as the protector of ethnic Russians beyond its borders, Milosevic’s and Tudjman’s divvying-up of Bosnia on behalf of its respective Serb and Croat populations or, indeed, Western support for the Bosnian Muslims and Kosovar Albanians in the same conflict, the use of the supposedly mistreated ethnic minority as a casus belli is a tried and tested tactic. This proved to be true in the case of the Sino-Vietnamese War, with Beijing accusing Hanoi of mistreating the ethnic Han Hoa population within Vietnam. Like many instances of this excuse, the actual mistreatment was hyperbolized; indeed, agents provocateurs from the Chinese embassy leaned on the Hoa press to print anti-Soviet (and, with the chess match of the Sino-Soviet split in mind, implicitly anti-Vietnamese) tracts. Nor was Hanoi’s treatment punitive—it was focused on trying to assimilate them more deeply into Vietnamese culture. Hoa sufferings, to the extent they existed, were a pretext.

4. Redefining the objective if the original no longer works.

Most of the Washington foreign-policy establishment knows a slight of hand when it sees one. When Barack Obama announced in August 2012 that the deployment and use of chemical weapons by Syrian forces still loyal to Bashar al-Assad would constitute the crossing of a “red line,” the phrase was suitably ambiguous for the president to fill in at a later date a precise explanation of what would occur if the line were crossed—though most (including Obama, according to Jeffrey Goldberg’s study of the president’s decision-making) assumed it would involve airstrikes. When that proved to be politically infeasible, John Kerry, in taking a reporter’s question regarding whether commandeering Assad’s existing chemical weapons stockpile with Russian assistance would fulfil the action necessitated by having drawn the red line in the first place, suddenly found his out. This was the foreign-policy equivalent of moving the goalpost and claiming the match.

The Chinese had to use the same move once they discovered just how intractable their Vietnamese adversaries were. With two hundred thousand troops were committed to the venture, a further million mobilized, and Deng personally seeking Jimmy Carter’s assurance that the United States would not interfere in the forthcoming war, there was every indication that Beijing had far more ambitious goals in mind. China’s admission that it was intervening in order to aid its Cambodian ally would lead one to believe that it intended to fight on until actionable progress had been made on the Cambodian front. Yet three weeks later, once it was established that the Vietnamese would neither quit Hanoi nor remove any forces from Cambodia to counter the northern threat, Beijing began to hedge its rhetoric, claiming that proving the Soviet Union incapable of defending its ally was a victory in itself.

With tension rising in the South China Sea and the U.S. arms restrictions dialling back, the Sino-Vietnamese War deserves another look.

Matthew Pennekamp is a resident junior fellow at the Center for the National Interest.