She wrote in 2000 that the Hague Convention must have power to enforce a custody order that "by its very nature limits each parent's unilateral decision-making power, including his or her power to relocate to another country with the child."
In an unrelated 2010 high court case, Justice Sotomayor succinctly summarized the delicate balancing courts around the world must maintain.
"The purpose of the convention is, which court will decide the life of that child," she said. "And to avoid, as I understood the convention structure, this flight from court to court and this long, drawn-out process from country to country over who's going to make that choice."
Both sides agree clarity on the issue is needed, if not for them, then for other parents in similar situations.
"Ultimately what's at the heart of the treaty is the welfare of the child," said Cullen. "And it's not good for a child to be like a ping pong ball going backwards and forwards between different countries. And that's why the treaty solves a problem."
As for the Chafins -- still technically married but apparently never again to be husband and wife -- both share a measure of regret over their broken union and a child caught in the middle, as well as anxiety over what the high court will do.
"I believe that if my daughter's returned to the U.S., I will never see her again," said Lynne Chafin, who said this fight is about Eris' future and well-being.
Jeff Chafin is equally determined. "I'm going as far as I can. Until I humanly, possibly cannot go any further, I will not stop fighting for my daughter."
The case is Chafin v. Chafin (11-1347). A ruling is expected by the spring.