HONG KONG — In 1840, Britain went to war with China over questions of trade, diplomacy, national dignity and, most importantly, drug trafficking. While British officials tried to play down the illicit origins of the conflict, opponents gave it a name that made the link quite clear: the Opium War.

The war’s settlement forced Chinese ports open and gave Hong Kong to Britain. It began what China calls the “Century of Humiliation,” when foreign powers forced weak Chinese governments to cede territory and sign unequal treaties. Britain and France waged a second Opium War against China from 1856 to 1860. China’s current leader, Xi Jinping, alludes to the era in his call for a “China Dream” of national rejuvenation.

The war is often seen as having been inevitable. But viewed through the lens of its own era, the conflict is deeply counterintuitive, Stephen R. Platt writes in “Imperial Twilight: The Opium War and the End of China’s Last Golden Age.” The new book from Mr. Platt, a history professor at the University of Massachusetts, Amherst, explores how the war came about through the influence of unscrupulous traders.

In an email interview, Mr. Platt discussed the origins of the Opium War and its influence on China’s relations with the world today.

It was surprising to learn about the extent of public opposition to the war in Britain. How was it able to go forward?

Yes, the war was incredibly controversial in its own time, far more so than I expected when I started my research. For the proponents, it was a matter of framing. They denied any connection to opium and argued that the war was entirely about defending Britain’s national honor and protecting their countrymen from alleged atrocities in China. But the involvement of opium was inescapable — thus the name “Opium War,” as the London Times and other papers called it. To many people in Britain the notion of going to war to advance the interests of drug dealers, against a country that had always been friendly to Britain, was abhorrent. As William Gladstone wrote in his diary at the time, “I am in dread of the judgments of God upon England for our national iniquity towards China.”

The collision between those two sides came to a head in the spring of 1840 with a huge debate in the House of Commons over a motion to stop the war by forcing the resignation of the ministers who started it. After three full nights of debate, with impassioned speeches that in some cases went on for hours, the motion failed by a razor’s margin.

How did China and Britain’s lack of mutual understanding influence the conflict?

Sadly, in some ways it worked better the less they knew about the other. When China was still a mystery, it was seen as unified and impenetrable. As the British started learning the reality of conditions in the empire, however, it became apparent that it was weaker than imagined and there were serious divisions within its society. On its face, the Opium War was almost absurd in its conception: the British sent a small fleet and a few thousand troops to make war on an empire of more than three hundred million people. But they were emboldened by reports from travelers that the merchants of China wanted free trade with the British and only their government stood in the way — essentially, that the British would be welcomed by the ordinary people with open arms. It was a gamble that would have been unthinkable a generation earlier.

You also show how people who learned about the other’s culture were better able to interact, like the missionary and interpreter Karl Gutzlaff. But the person in Britain who had perhaps the most knowledge of China, George Staunton, was key to the war being launched. What does this say about the value of such knowledge?

They didn’t always use their knowledge for good ends. Gutzlaff, for instance, was one of the most talented linguists of his age and he wound up interpreting for opium smugglers. But in a broader sense, the events of this era are a reminder that so-called experts do not always appreciate the limits of their own knowledge. When the country they profess to understand so well behaves in ways they think it shouldn’t, they can become especially hostile critics. It’s almost as if they feel personally betrayed.

In Staunton’s case, he was vocally opposed to the opium trade and had acted as Britain’s voice of conscience towards China in the past. If this had been a movie then he would have stood up in the House of Commons in 1840 and denounced the war and everyone who supported it. But he did exactly the opposite. As a historian it was heartbreaking to see him do that, but that is one of the things that makes history so fascinating. Sometimes people just don’t do what you expect, and when that happens it opens up a whole new dimension of their character.

Britain’s early diplomacy with China introduced the word “kowtow” to the English language, from the Qing court ceremony of prostrating before the emperor. You write that debates over the kowtow and their supposed effect on future relations are not clear-cut. How so?

As the British saw it, the kowtow was a national humiliation — basically, their ambassador was being asked to abase himself before China’s emperor. It became for them the ultimate symbol of Chinese arrogance and inflexibility. The kowtow even became a sort of hindsight logic for the Opium War: Britain had to fight that war, the reasoning went, because the Chinese refused to treat Westerners as equals. The irony of this is that actually neither of Britain’s ambassadors to China before the war were refused audiences for declining to kowtow. The Qing court showed itself to be more flexible on this count than the British. Which is to say that the hysteria about the kowtow really says more about Great Britain than it does about China. In any case, some Western observers at the time wondered why the British should expect China to adapt its court ceremonies just to suit them. As Napoleon put it, if it was the custom of the British to kiss their king on his buttocks, would they go to China and demand that the emperor drop his trousers?

As we enter into a period of increasing tension between the United States and China, particularly over trade, are there any lessons to be learned from two centuries ago?

In the early nineteenth century, trade was a common language between China and Britain despite the great differences in their national cultures. Chinese and British officials alike recognized that the legal, aboveboard trade was a strong stabilizing factor in international affairs. It was when governments intruded too directly, and especially when issues of national prestige entered the mix, that problems would arise. Left to its own devices, however, the Canton trade was a largely peaceful and profitable meeting of civilizations. So maybe the lesson to remember today is that economic engagement provides the ballast for our relationship with China, and we should be very careful how we let politics interfere with it.