It was born out of American and British intelligence collaboration in World War II, a long-private club nicknamed the “Five Eyes.” The members are five English-speaking countries who share virtually all intelligence — and pledge not to practice their craft on one another. A former top U.S. counter-terrorism official called it “the inner circle of our very closest allies, who don’t need to spy on each other.”

This is the club that German chancellor Angela Merkel and French President Francois Hollande say they want to join — or at least, win a similar “no-spying” pact with the U.S. themselves.

It all began with a secret 7-page agreement struck in 1946 between the U.S. and the U.K., the “British-US Communication Agreement,” later renamed UKUSA. At first their focus was the Soviet Union and its Eastern European satellites. But after Canada joined in 1948, and Australia and New Zealand in 1956, the “Five Eyes” was born, and it had global reach. They pledged to share intelligence — especially the results of electronic surveillance of communications — and not to conduct such surveillance on each other. Whiffs of the club’s existence appeared occasionally in the press, but it wasn’t officially acknowledged and declassified until 2010, when Britain’s General Communications Headquarters, or GCHQ, released some of the founding documents. The benefits of membership are immense, say intelligence experts. While the U.S. has worldwide satellite surveillance abilities, the club benefits from each member’s regional specialty, like Australia and New Zealand’s in the Far East. “We practice intelligence burden sharing,” said one former U.S. official. “We can say, ‘that’s hard for us cover, so can you?'” The ease and rapidity of information-sharing among the five “makes it quicker to connect the dots,” said another intelligence veteran. “You can’t underestimate the importance of the common language, legal system and culture,” said another. “Above all, there is total trust.”

That trust extends to not tapping the phones of one another’s leaders and officials. That’s rooted in the belief that when their leaders talk to one another, they do so in full candor. “There is very little we need to know about these countries and their leadership’s views that the leaders wouldn’t tell us themselves, with all honesty,” said a retired official familiar with the program.

A murkier question is whether they’ve also agreed never to spy on each other’s citizens. U.S. officials say that’s part of the deal. Yet there have been reports in the British press — amplified most recently by former NSA contractor and leaker Edward Snowden — that that’s not the case, that the Five Eyes spy on one another’s citizens and share the information to get around laws preventing agencies from spying on their own citizens. Former CIA deputy director John McLaughlin insisted to me that this isn’t so. “I’ve never heard of that,” he said. “You would think I would know if that were the case.”

But can we say for sure that unlike Merkel, British Prime Minister David Cameron doesn’t have to worry that his cell phone is bugged? Said a longtime spymaster, “Not by us.”