In 2005, he spoke for a 5-4 majority in abolishing executions for murderers younger than 18 at the time of the crime. Under the standards of the nation and the world, imposing the death penalty on young murderers is cruel and unusual punishment, he said.
"It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty. . . ." he wrote in Roper vs. Simmons. "The opinion of the world community, while not controlling our outcome, does provide respected and significant confirmation for our own conclusions."
Kennedy came under sharp criticism from Republicans on Capitol Hill for that opinion. They talked of impeaching justices who rely on foreign law as a basis for their decisions.
Kennedy was not speaking for himself alone Thursday.
Justice John Paul Stevens, a World War II veteran, also has argued that American law must deal fairly with enemies as well as friends. He wrote a 2006 opinion, along with Kennedy, which held that basic protections of the Geneva Convention extend to those who are captured and imprisoned during the war on terrorism.
Joining them in the majority Thursday were Justices Stephen G. Breyer and Ruth Bader Ginsburg. They travel widely and meet often with foreign judges and international legal groups. They too say American law should follow the basic norms of international human rights law.
Justice David H. Souter's summer travel usually consists of driving home to New Hampshire. He avoids judicial conferences and nearly all requests to speak to legal gatherings. But he has regularly joined Kennedy and the others, as he did Thursday.
In a brief opinion to challenge Scalia's heated denunciation, Souter wrote: "The several answers to the charge of triumphalism might start with a basic fact of Anglo-American constitutional history: that the power, first of the crown and now of the executive branch of the United States, is necessarily limited by habeas corpus jurisdiction to enquire into the legality of executive detention."