“To sum up what distinguishes the United States in a nutshell: It’s the First Amendment,” said Steven Aftergood, the director of the Project on Government Secrecy at the Federation of American Scientists. “The concept of a free press has been integral to the American idea since its inception. That’s not true even of other democracies. The press here even has the right to be irresponsible, which it sometimes is.”

The contrast with Britain, despite the shared democratic heritage, is particularly stark. Instead of the First Amendment, the British have the Official Secrets Act, which allows the government to ban in advance the publication of government secrets and prescribes punishments not just for leakers, but also for the journalists who publish the information.

Despite an unprecedented string of prosecutions for leaks under the Obama administration and a pledge on Thursday by Mr. Trump’s attorney general, Jeff Sessions, to end “these rampant leaks that undermine our national security,” unauthorized disclosures of secrets are far more common in Washington than in London.

“I’m trying to think of a scandal over a leak from the intelligence service here, and I can’t think of one,” said John Lloyd, a veteran British journalist and a founder of the Reuters Institute for the Study of Journalism at Oxford University. “The culture of ‘you don’t need to know this’ hangs around in the U.K.”