- State officials, in a previous federal suit on a federal cause of action, signed a consent decree;
- The officials were properly parties (instead of the state itself) under Ex parte Young; and
- A plaintiff later files suit against the state itself (not the officials, who may well no longer be in office) to enforce the consent decree,
(Aside: I realize that the last sentence is legalistic and relies upon technical vocabulary. "Modify" could have been just "change"except that this would make the conditional clause ambiguous without adding a qualifier to each use of the word "change." This is one of the few times that a legalism actually makes things clearer; "modify" isn't that difficult a word, even in the legal context.)
This is the correct decision. IMNSHO, Eleventh Amendment immunity has been extended too far, with College Savings Bank being the most-obvious example. It is also logically inconsistent with Article V, but does not explicitly modify (there's that word again) Article Vit has been treated through the years only as modifying jurisdiction under Article III. There's that "read the Constitution as a whole" argument again! Even without this, though, the key question is not actually one of immunity; it is, as Justice Kennedy's opinion makes clear, one of enforcing something already agreed to in a fashion that does not place form over substance. We all pretend under Ex parte Young that suing a government official in his/her "official capacity" is not the same thing as suing the government itself. This explains, for example, why last year's copyright-extension decision is Eldred v. Ashcroft (previously filed as Eldred v. Reno), not Eldred v. US. Justice Kennedy's decision recognizes that this distinction really makes no sense when it comes time to enforce a judgment obtained against those officials when they later (allegedly, anyway) violate that judgment. That is what a consent decree is: it is a settlement of a lawsuit and thus has the same force as a verdict from a jury trial.