In fact, Britain's foreign secretary has long been a dual national. Johnson was not only a British citizen, but also, thanks to being born in New York City, a natural-born U.S. citizen, too.

Until recently, at least.

As the Wall Street Journal first reported, Johnson's name was included on a list of 5,411 people who renounced their U.S. citizenship during 2016. The British foreign secretary is included in the document, put out by the U.S. Treasury Department on Wednesday, under the name “Alexander Boris Johnson,” an abbreviated form of his full birth name, Alexander Boris de Pfeffel Johnson.

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Johnson has not mentioned his decision publicly, and there was no immediate response to an inquiry at the British Embassy in Washington. But even before he became Britain's top diplomat, it was well-known that Johnson's dual citizenship was causing him a headache.

During interviews conducted in the midst of a stateside trip in 2014, Johnson was repeatedly asked about one of the downsides of dual citizenship when it comes to the United States — dual tax systems. Speaking on WAMU's Diane Rehm show, Johnson said it was unfair that the U.S. government was forcing him to pay capital gains tax on the sale of his Islington home, even though he had not lived in the United States since he was 5 years old.

“Can you believe it?” Johnson told Rehm. “I think it's absolutely outrageous.”

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Johnson may have had a point. The United States is one of only two countries in the world that requires a citizen living abroad to file tax returns each year (Eritrea is the other), a policy based on a Civil War-era law designed in part to punish Union Army deserters. Most of the time, this works out fine, as the United States would not require you to pay tax on something you had already been taxed on.

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However, there are times when tax systems don't line up. And in Johnson's case, the difference in the two tax systems proved expensive.

In Britain, you are not required to pay taxes on the sale of a home, yet in the United States, you must pay a capital-gains tax on a gain above $250,000. That figure is far below what the sale of a large home in North London would net you, and though U.S. homeowners would have been subsequently able to claim a deduction, Johnson may not have been. Some estimates put Johnson's U.S. tax bill at six figures.

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For expats and U.S. green-card holders living abroad, these tax rules are well-known and the subject of much acrimony. Worse still, a 2010 law called the Foreign Account Tax Compliance Act (FATC) forced foreign banks to help claim back tax money from U.S. citizens living abroad, making it easier to collect unpaid taxes. It's thought that this new law is one of the primary reasons thousands of people renounce their citizenship or green card each year.

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Johnson had threatened to give up his U.S. passport a number of times — in fact he wrote an entire article about it in 2006. But instead he kept hold of it for years for unclear reasons, even renewing his U.S. passport in 2012, according to some reports. Early in 2015, the British media reported that he had finally caved and agreed to pay his U.S. tax bill.

It's unclear why Johnson decided to give up his U.S. citizenship last year, though it may well have been his elevation to the role of foreign secretary last summer that made him take the plunge. In this role, Johnson was recently forced to push back on a U.S. administration whose executive orders had caused confusion among British dual nationals.

There's another possibility, too. While there is no rule against it, no previous British leader had been a dual national. Johnson's decision to forsake his American passport may open up that door to him, though it will also close another possibility he once discussed: becoming U.S. president.