On the eve of becoming Britain’s ambassador to the United States, Peter Mandelson had a busy Monday morning as he packed up his house and made his way to Buckingham Palace for a meeting with King Charles III.

However, the strain of departing London might not be as great as what Mr. Mandelson will face upon his arrival in Washington on Wednesday. Not many British ambassadors have accepted a position as politically risky as Mr. Mandelson’s. Some of America’s strongest ties are already in jeopardy, and his first day at the embassy will fall on President Trump’s eighteenth day in the White House.

Mr. Mandelson was packing boxes while monitoring Mr. Trump’s recent interactions with Canada and Mexico following the imposition of sweeping tariffs, which he later suspended. His next target appeared to be the European Union. Although he said that Britain’s trade balance with the United States was “way out of line,” Mr. Trump was kinder to the country, telling reporters that a settlement “could be worked out.”

Using a scrupulously polite tone, Mr. Mandelson stated in an interview, “I’m not going to tell the president his business when it comes to trade.” “We have a balanced trade relationship with the U.S.,” he asserted. Both the commodities and the services are balanced.

The challenge for Mr. Mandelson will be to keep Britain out of Mr. Trump’s sights, especially at a time when the left-of-center administration of Prime Minister Keir Starmer is attempting to mend the country’s long-standing animosity toward the European Union.

Although they were unable to stop Mr. Mandelson’s nomination, some Trump supporters have already condemned him, which may have contributed to his decision to apologize on Fox News last week for calling Mr. Trump a “white nationalist” and a “danger to the world” during his first term.

Mr. Mandelson is well-versed in trade flows, having led Britain’s Board of Trade until 2010 after serving as the European Union’s trade commissioner from 2004 to 2008. He expressed his optimism that his background would enable him to provide a convincing argument for Britain, which, depending on whether one uses American or British data, has a $14.5 billion trade deficit or a $89 billion trade surplus with the United States. (How the two sides handle royal dependencies like Jersey and Guernsey, which are offshore financial hubs, is one factor contributing to the discrepancy.)

He declared, “I will use that knowledge to promote some mutual understanding.” “I’m not going to be front-of-house in making arguments against the president’s policies,” he quickly added. It is my responsibility to operate behind the scenes and inform one another of our nations’ policies.

Given the president’s propensity of running afoul of prominent figures, keeping a low profile makes sense in Mr. Trump’s Washington. However, it is inconsistent with Mr. Mandelson’s character. Throughout his four-decade political career, he frequently brought himself into the public eye. Known for his brutal methods as a young Labour Party strategist, he was dubbed the Prince of Darkness.

The 71-year-old Mr. Mandelson has since been in and out of favor with many Labour leaders, ranging from Mr. Starmer, with whom he got close prior to his election triumph last July, to Tony Blair, for whom he was once a trusted adviser.

Mr. Trump received some of those elbows. Apart from his remarks in a 2019 interview with an Italian journalist, Mr. Mandelson criticized Mr. Trump for placing tariffs on China in a 2018 piece for the London newspaper The Evening Standard. He claimed that the president was a “mercantilist and a bully.”