This is why the Cameron government put joint efforts against tax avoidance and corruption at the top of the agenda when it chaired the Group of 8 summit meeting back in 2013. Even the watered-down final declaration still looks groundbreaking. Information about company ownership should be made available to the authorities (hello, Delaware and the British Virgin Islands!). Companies should stop shifting money across borders to minimize their tax bills (Apple beware). Poor countries should be helped to obtain the taxes they’re owed (from the trade in bananas, say).

The hope was that tackling the misuse of the financial system and corruption in developing nations would help solve many of the world’s most pressing problems: poverty, economic stagnation, illegal immigration, even terrorism. “Corruption adds 10 percent to business costs globally,” Mr. Cameron said in a speech in Singapore in July 2015, and “cutting corruption by just 10 percent could benefit the global economy by $380 billion every year.”

Even while the referendum loomed, and Britain’s fate teetered on the thin margins of opinion polls, Mr. Cameron maintained his focus on this global problem. In May last year, the month before the Brexit tsunami capsized his premiership, he hosted an anticorruption summit in London and won commitments from dozens of countries to do more to open up their economies and combat financial crime.

But then the Brexit referendum happened, Britain turned inward and all that stopped. The Cameron government had promised to publish its anticorruption strategy by the end of 2016. No strategy has yet emerged. As Jon Benton, who headed the National Crime Agency’s International Corruption Unit and worked in the cabinet office in the months before the summit in May 2016, told me earlier this month: “When Theresa May took over, the anticorruption phone just stopped ringing.”

Instead, we had the Conservative Party’s manifesto for the entirely unnecessary election that Ms. May fluffed earlier this year. It does not mention the word “corruption” once, and contains just a few bromides about tax avoidance. It also pledges to strip the Serious Fraud Office of its independence, which is an inexplicable response to that office’s winning some of the biggest victories against corporate corruption in British history and pushing for more, largely thanks to a legal reform introduced by Mr. Cameron’s government.

Brexit didn’t just doom the prime minister’s career, it also doomed the campaign he was leading to improve the world.