It’s easy to hold grudges; it envelopes one in a sense of superiority, a feeling of wronged righteousness, that allows irrational behavior to feel very, very good. When someone hurts you, it can be wonderful to lord that over them forever.

Few wars in American history involve as many hurt feelings as the Vietnam War. Depending on when you chart it, the war lasted anywhere from the late 40s to 1975, when North Vietnam conquered the South. For the US, earnest combat began in 1965 and lasted until 1973, when the Nixon Administration washed its hands of Southeast Asia.

The toll was hefty: 58,000 Americans and anywhere from 1.4 to 3.8 million Vietnamese died. On the American side of the Pacific, the war gave counterculturalism a salient boost in the body politic and for decades much of American foreign and domestic policy hung on the legacy of those years. In Vietnam, the regime used fear of another American invasion to build legitimacy — and support war in Cambodia — up until the 1990s.

So it would be rather easy for both sides to neither forgive nor forget. Whole careers could be made off holding a grudge.

And yet the Americans are about to start arming the Vietnamese.

What the hell just happened here?

A key geopolitical principle: people hold grudges until they can’t anymore

In the case of Israel and Palestine, the grudge match almost wholly defines the political nature of both states. That may seem like history is the definer of the future, but the reality is quiet different.

Geopolitical grudges and resentments are often used by elites to gain leverage in their own domestic politics: Leader A feels insecure so they stir up memories of some foreign horror, reminding people that only Leader A stood up to the bad guys. America’s Republicans used to have this down as an art form: only Republicans were tough enough on America’s enemies. (Donald Trump has muddled that formula, as he has so much else within the Republican Party.)

But that gains only so much traction and works only for so long. Invariably, the political capital gained is outpaced by the geopolitical conditions of a nation state. In other words, you can dredge up the war you once fought with your neighbor all you like, but people eventually stop listening, let alone caring, as conditions shift.

After all, Britain and France spent a good deal of time and energy butchering one another all other planet Earth, yet as Germany emerged as a more powerful threat the policy gap between London and Paris closed rapidly. This too happened between Germany and the Western powers: once Germany lay prostrate and the Soviets emerged triumphant, the gaps between Berlin, London, DC and Paris all closed into NATO.

In other words, “can’t” often means being faced with either a greater threat and/or a greater opportunity than the grudge provides

In the case of many NATO states, some of whom were archenemies in the past (Germany and France, Greek and Turkey), it was the “greater threat” of Soviet communism that drove them together.

But there is also the case of greater opportunity. That can be economic, military or political. Mexico could, after all, readily get very upset about losing nearly half its territory to the United States, but the opportunities of cooperation with the United States — lower military costs, stronger trade ties, greater say in their own political affairs since overt American interference is unlikely — is much greater than clamoring on about what happened a century and a half ago.

Often, these combine and when they do, history really does consign itself to the dustbin. This is the case of Vietnam.

First, the economics

The American exports some $29.9 billion back. While it’s small potatoes for the United States, that’s some 17.5 percent of total Vietnamese GDP per year brought in from the United States. Nearby China, meanwhile, earns Vietnam less: only $17.5 billion. This makes a great deal of sense: both China and Vietnam are communists-turned-manufacturers and therefore do many of the same tricks, making them competitors. (Click those links for some great data visualizations courtesy of MIT.)

Such value in the American market softens Hanoi’s attitude toward its old foe. Its once ideological fellow traveler, China, is no longer a common market: in the dog-eat-dog world of international capitalism, drawing closer to the vast export market of the United States makes a huge amount of sense for any nation state trying to develop itself.

Then, the geography

The basic fact that America is far away makes any relationship considerably more comfortable. Should America ever get into an annexing mood, it will go for Canada, Mexico or some Latin American country long before Vietnam.

But China? Well, China will always live next door. And China has spent many a century ruling Vietnam. What’s to stop it from doing so again? This was a key reason why Hanoi, in the aftermath of victory in 1975, much preferred Soviet alignment to Beijing’s. It’s also one of the very basic reasons why Vietnam and China had a conventional war in 1979: a border is a simple thing to fight over. While there were proximal causes of the war, China’s location next to Vietnam means that future disputes between Hanoi and Beijing can escalate to war much faster than any dispute between America and Vietnam.

Then, the South China Seas

The Chinese building whole islands in the South China Seas is unnerving every nation state in the region. Thus far, these islands haven’t amounted to much: the Americans blithely sail right through their claimed waters to prove the point. But this relies upon American interest and resolve: should either waver, the Chinese could well bully the smaller nation states right out of the water.

What better way to build American interest than solidify an alliance, even an informal one? America’s overriding interest in being seen as a reliable ally means that it’s far less likely to abandon friendly countries wherever they may be. (Friendly regimes don’t get the same treatment; Americans, both elite and citizenry, think of governments and populations as two very different things, and are always happy to drop a government while proclaiming loyalty to a people.)

So the combination is there: it just took a while to flesh it out

After the collapse of the Soviet Union in 1991, Vietnam was left adrift, much like North Korea, in a sea dominated by American power. China had yet to emerge as the economic superpower we know today. In 1995, Vietnam and the United States began rebuilding their relationship: history be damned, the end of the Cold War meant the United States wanted friends wherever it could find them while Vietnam’s elite desperately needed to know America, now freed of the Soviet foe, would not return to try to finish the job.

As Vietnam, like China, developed into a major manufacturing hub under firm state control, the Vietnamese had little choice but to seek advanced Western markets. America is the gatekeeper: antagonizing it earns embargoes, as both Iran recently and Iraq under Saddam Hussein more distantly learned. While trade with China is important, the vast majority of Vietnamese exports go to American allies.

Vietnam’s ruling elite doubtless see regime security as paramount in the relationship: trade and friendship with America is not a door to democracy. The Americans see it differently; that’s exactly what trust and friendship with America should do. This will cause conflict over time when the expectations of both sides aren’t met, but so long as China remains a potent military force whose place in Asia’s security system isn’t totally clear there will be much more to unite them than divide them.

This story first appeared at Geopolitics Made Super, May 26, 2016.