[Kamq]

Aren't they still sovereign? They just might be sanction/fined by peers or kicked out of the union if they don't comply?

It's not like someone's going to declare war on Ireland and conquer them to force compliance the way a citizen breaking the law might be conquered by a police force.

[ben_w]

The winning move for a single round of the Prisoner's Dilemma is to defect.

The winning move for the Iterated Prisoner's Dilemma is tit-for-tat.

Enforcement mechanisms lead actors to behave more cooperatively, and everyone knows everyone is better off if everyone is cooperating.

[Kamq]

That doesn't make anyone in the game not sovereign

[ben_w]

The enforcement mechanism is a voluntary de jure loss of sovereignty, even though the de facto sovereignty to leave the enforcement mechanism remains.

[Kamq]

Ok, that makes more sense