WASHINGTON — Justice Stephen G. Breyer, a progressive force on the United States Supreme Court for more than two decades, advocates the courts taking into account foreign law. That’s the stuff of a good debate, befitting a justice who got to the bench courtesy of former President Ronald Reagan and the archconservative Senator Strom Thurmond.

His ascent was mostly thanks to the exceptional political skill and standing of a mentor, Senator Edward M. Kennedy of Massachusetts, who got resurgent Republicans to approve the liberal judge when they could have instead tapped one of their own for the seat.

We will revisit that story, which illustrates the way Washington used to work.

In Justice Breyer’s recently published “The Court and the World,” his third book since becoming a justice, he suggests the court should look abroad for guidance on some decisions because about 20 percent of cases have something to do with what happens outside the United States. This notion is anathema to conservative members of the court, including Chief Justice John G. Roberts Jr. and the most forceful advocate of the right, Justice Antonin Scalia.

Justice Breyer says that in an interdependent world, the court “must increasingly consider foreign and domestic law together,” noting that “more harmonizing, understanding and application of American law to foreign activities is not the same as American courts deciding cases on the basis of foreign law.” This applies particularly to national security cases in the era of terrorism. From the Civil War through the internment of Japanese-Americans in World War II, the courts have largely given the president a blank check in times of war. That has changed with issues such as the detention center at Guantánamo Bay, Cuba. Justice Breyer goes through the competing claims of security, civil liberties and foreign laws.