LONDON — He told the world to grow up and accept the challenge of climate change. He made fun of France’s neuralgic reaction to being elbowed out of a submarine deal with Australia by Britain and the United States. He even cleared up lingering confusion about how many children he has (six).
Prime Minister Boris Johnson of Britain cut a characteristically colorful swath from New York City to Washington this week, managing to travel between the cities by Amtrak — a nod to his Amtrak-loyal host, President Biden — before telling the aggrieved French to “prenez un grip” and “donnez-moi un break.”
For Americans, now used to a president who rarely strays from the script, it was a throwback to a time when their own leader would show up in Britain and start lobbing cherry bombs. Except in the case of Donald J. Trump, that involved calling London’s mayor a “stone-cold loser” and telling a British tabloid that Mr. Johnson’s predecessor, Theresa May, was doing a bad job of negotiating a Brexit deal.
Mr. Johnson has always been a more genial, upbeat figure, a journalist-turned-politician who uses humor, often at his own expense, to make serious points. What is less clear, after a five-day visit that featured signs both reassuring and problematic for the “special relationship,” is how the prime minister’s lighthearted style advances Britain’s effort to stake out a post-Brexit role on the global stage.
“This is both Boris Johnson’s advantage and problem,” said Jonathan Powell, a former chief of staff to Prime Minister Tony Blair. “He’s great fun the first time you meet him. But the problem with being a comedy act is that you’re then not taken seriously. That’s why we weren’t consulted on Afghanistan.”
Britain’s inclusion in a nuclear-powered submarine alliance with Australia and the United States was a notable victory for Mr. Johnson — one that showcased Britain’s relevance and compensated for the White House’s disregard of British views on the tactics or timing of the military withdrawal from Afghanistan.
Still, it is a bright spot in a trans-Atlantic relationship that is otherwise a mixed bag. On the way to New York, Mr. Johnson told reporters that Mr. Biden had little immediate interest in negotiating a trade deal between the United States and Britain because he had “a lot of fish to fry.” Though hardly a surprise, his admission effectively buried one of the main selling points of Brexit: that it would enable Britain to cut a lucrative trade deal of its own with the United States.
With Mr. Johnson seated next to him in the Oval Office a few days later, Mr. Biden also made clear that he would object to any British actions that threatened peace in Northern Ireland. Britain has vowed to overhaul its post-Brexit trading arrangements with the north, a process that critics say could jeopardize the Good Friday Agreement, which settled decades of sectarian violence there.
British officials said Northern Ireland did not come up in their private talks, which one official characterized as “very warm.” But Mr. Biden’s public reference to it was a reminder that the issue has political resonance in Washington, and hence, continuing potential to disrupt the relationship between London and Washington.
Prospects for a bilateral trade deal have now been replaced by hopes for something arguably even more far-fetched.
British papers reported that the Johnson government is now mulling whether it could enter the revised North American Free Trade Agreement negotiated by Mr. Trump with Canada and Mexico. Since Britain already has deals with both countries, that would amount to a backdoor deal with the United States.
Trade analysts were puzzled, noting that this would not spare neither side the political hazards of a trade negotiation. Moreover, these experts said, the language in that deal, known as the United States-Mexico-Canada Agreement, would be disadvantageous to British automakers eager to export to the United States.
“Everything that makes a bilateral agreement difficult makes U.S.M.C.A. difficult,” said Sam Lowe, an expert on trade at the Center for European Reform, a research institute in London. “We’d still be talking about chlorinated chicken,” he added, referring to disputes over access for chemically treated American food.
For Mr. Johnson, the complexities of a trade pact may matter less in the short term than the wins he scored. On the eve of his visit, the White House lifted a ban on travelers from Britain, the European Union and other countries, which had become a nagging source of trans-Atlantic strain.
Mr. Johnson also got to crow over the submarine alliance, which not only makes Britain a key American ally in the geopolitical contest with China, but also has the political fringe benefit of vexing Britain’s neighbor, France.
Speaking outside the Capitol, Mr. Johnson broke into gleeful Franglais to poke fun at the French for what he said was their overreaction to Australia’s decision to break a $66 billion deal for non-nuclear submarines.
“Donnez-moi un break” became an instant classic on social media, rivaled only by a moment, in an interview with NBC News, in which Mr. Johnson admitted to having six children. The precise number has long been cloaked in mystery: He has been divorced twice, has a daughter through an extramarital relationship, and has evaded previous attempts to pin him down on the paternity question.
As seasoned Johnson observers noted, he has used the Franglais version of “give me a break” at least eight times, going back to March 1994, when he put it in an article about housing prices. Some critics argued it was needlessly provocative to France, getting a laugh out of a country that has plenty of ways to settle scores with Britain.
“We enjoy the times the French get hot under the collar,” Mr. Powell said. “But there is a long-term cost to that.”
At the United Nations, where Mr. Johnson is not yet a familiar fixture as a world leader, he deployed a trademark mix of charm and self-deprecation. He told reporters that as a journalist, he had played down the threat of a warming planet. Speaking to the General Assembly as the host of a United Nations’ climate conference in Glasgow, Scotland, in November, Mr. Johnson slipped into the role of an affectionate but stern parent.
“We still cling with parts of our minds to the infantile belief that the world was made for our gratification and pleasure,” Mr. Johnson declared, in words that could apply to his own picaresque past. “And we combine this narcissism with an assumption of our own immortality.”
“We believe that someone else will clear up the mess, because that is what someone else has always done,” he added. “My friends, the adolescence of humanity is coming to an end and must come to an end.”