(CNN)Britain briefly stopped sharing intelligence with the United States about the Manchester bombing after American officials leaked information -- a remarkable development between two Five Eyes nations that have the deepest of intelligence relationships.
What is the Five Eyes intelligence pact?
The US and the UK are two of the so-called Five Eyes -- along with Canada, Australia and New Zealand -- that share a broad range of intelligence in one of the world's tightest multilateral arrangements.
The group has been in the spotlight several times this year, including when the US' partners felt compelled to publicly back it amid reports that US President Donald Trump shared top secret information with Russia.
Here is a look at what the Five Eyes arrangement is, and why it stands out from other intelligence-sharing deals across the world.
The United States and Britain had a smooth intelligence relationship in World War II, and they formalized it after the war with the BRUSA (later called UKUSA) Agreement in 1946.
Helping to bond the allies was a joint effort to decrypt Soviet intercepts that helped reveal that spies had compromised the US Manhattan Project to build an atomic bomb, said Kristian Gustafson, senior lecturer of intelligence studies at Brunel University London.
"There was in the United States a creeping realization that maybe the Russians weren't going to be their friends in the 1950s," Gustafson said.
As former UK dominions exercised greater sovereignty, Canada (1948) and Australia and New Zealand (1956) began representing themselves in the intelligence pact.
There are other multilateral intelligence-sharing arrangements, such as within NATO, but more information gets shared among the Five Eyes, bonded in part by a common language and decades of trust.
"Even inside NATO, nobody shares everything," in part because there are too many nations with interests that sometimes differ, Gustafson said.
The United States also maintains intelligence-sharing relationships with allies like France, Germany and Japan. In the Middle East, the US formally and informally shares information with several countries in the fight against ISIS and other terror groups. They include Jordan, Egypt, Saudi Arabia, the United Arab Emirates and Turkey, among others.
But Five Eyes members keep some things to themselves, and share with others on a case-by-case basis.
Two years ago, for instance, the Five Eyes agreed to share with France some of their most sensitive intelligence on ISIS in Syria because of the November 2015 Paris terror attacks, a US official told CNN on condition of anonymity at the time.
That intelligence was "previously shared only with the Five Eyes countries," the official said.
Many other relationships tend to be bilateral and based on barter -- one side gives information in exchange for something else.
But Five Eyes is "more of a trust economy -- you can go to a bar and order a drink on credit," Gustafson said.
Helping to make it work: The five nations have similar security standards and classification systems.
Britain late Thursday resumed intelligence sharing in the Manchester investigation. "Fresh assurances" ensure that "we are now working closely with our key partners around the world, including all those in the Five Eyes intelligence alliance," said Mark Rowley, assistant commissioner of the National Police Chiefs' Council.
British officials had fumed about the Manchester leaks. At a NATO summit in Brussels, UK Prime Minister Theresa May affirmed Thursday that the UK's intelligence sharing was "our deepest defense and security partnership" and said she'd confront Trump about the leaks.
The UK's disappointment stems from a principle that helps make Five Eyes work -- a deep trust that if one provides another with data, the recipient won't reveal it without the originator's permission, Gustafson said.
Trump said he'd ask his administration to "launch a complete review of this matter and, if appropriate, the culprit should be prosecuted to the fullest extent of the law."
"There is no relationship we cherish more than the special relationship between the United States and the United Kingdom," the President said in a written statement.