It began as two nations reading one enemy's mail. At Bletchley, in 1941, before America was even in the war, British and American codebreakers started trading what they had — the broken Enigma, the cracked Japanese ciphers. They formalized it in the BRUSA agreement of 1943, and then, in March 1946, in a secret treaty called UKUSA that bound American and British signals intelligence into a single system. Canada came in by 1948, Australia and New Zealand by 1956. Five nations, one ear laid against the world.

Begin with the honest half, because the critics rarely do: it was necessary. The codebreaking shortened a world war and saved a number of lives no one can count, because they were never lost. The Cold War then put a nuclear adversary on the far side of the earth, and the only way to watch a closed empire that could end the world in an afternoon was to divide the planet into sectors and listen — continuously, jointly, forever. For eighty years the Five Eyes has been the quiet spine of Western security, and a history that leaves that out to get to the outrage faster is a child's history. The alliance is not a conspiracy. It is an instrument, and it worked.

The instrument had a shape. Signals intelligence first — the electromagnetic take of the planet, divided by geography and shared across all five so that each saw what any one collected. Through the Cold War they built ECHELON to sweep Soviet and Eastern Bloc traffic wholesale. In time the take widened past signals into military, human, and geospatial intelligence — an apparatus built, in principle, to see everything an enemy said or did.

But an apparatus built to see everything does not, in the end, keep its aim on the enemy. That is the cost, and it arrived slowly and in public. Duncan Campbell described ECHELON in 1988; Nicky Hager documented it in 1996; the European Parliament convened a committee on it in 2001. Then, in 2013, Edward Snowden's disclosures laid the modern scale bare — PRISM, XKeyscore, Tempora, MUSCULAR — and the ear that had been trained on Moscow was plainly trained on the phone in an ordinary person's pocket. The machine did not turn inward out of malice. It turned inward because that is what a machine built to see everything does when the enemy it was aimed at dissolves and the capability remains.

And here is the sharpest cost, the one that is structural rather than incidental: the swap. Each of these five governments is restrained, at home, by its own law from freely spying on its own citizens. The alliance quietly dissolves that restraint. You cannot lawfully watch your own people — so your ally watches them and hands you the product, and you watch your ally's people and hand back theirs, and no single government has technically broken its own law. Liberty's director called it subcontracting the dirty work. In 2013 a Canadian federal judge rebuked his own country's service for outsourcing the surveillance of Canadians to partner agencies and called the practice illegal. The structure launders the transgression: the citizen is watched, the law is intact on paper, and the leash each free society keeps on its own spies is slipped by the simple expedient of borrowing someone else's hands. The alliance built to defend the open society became the mechanism by which each open society evades the limits that make it open.

Then the secrecy, which is its own cost. UKUSA was signed in 1946 and not released to the public until 2010 — sixty-four years during which the most consequential intelligence arrangement of the age operated beneath the reach of the voters it claimed to serve. Democratic accountability did not lag the apparatus by a step. It lagged by two generations, and only caught up because journalists and a leaker forced it to.

So again: necessary, and the cost is great. The alliance is a genuine instrument of a real defense, and in the same motion it built a machine that can watch the world and a loophole that lets five governments slip their own restraints against watching their own. Both are true, and they are not in tension — they are the same fact seen from two sides. The eye that guards you is the eye that reads you, and there is no arrangement on offer where you are given the first without the second. That is the toll of the watch. And like most tolls of this kind, it is paid mostly by people who are never told there was a charge.

Sources: Five Eyes, Wikipedia; Temporary Committee on the ECHELON Interception System, Wikipedia.