The so-called five-eyes grouping of the US, UK, Canada, Australia and New Zealand is in the news and increasingly referred to as an alliance. That designation overstates the case somewhat. The grouping began as a UK-US signals intelligence sharing arrangement during World War II and expanded following the end of the war. This arrangement has both remained true to its roots and evolved considerably since then.
Close cooperation between the electronic intelligence agencies of the five countries continues. This arrangement allows for more or less seamless tasking and sharing of each other’s capabilities. The original signals sharing arrangement has inevitably spread to the rest of the intelligence community. There are almost equally close links between the human and defence intelligence agencies of the five countries. At this level the arrangement is primarily a technical and operational (as opposed to strategic) one, with considerable benefits to all partners. It must be noted, however, that ‘operational’ can conceal a very close relationship indeed. This may mean that there is a risk agencies could become complicit (if unwittingly) in the illicit activities of partner agencies, although legislation and policies are in place to prevent that from happening.
But, as news headlines routinely remind us, it would be a mistake to consider this as 'merely' an intelligence relationship.