A bitterly divided U.S. Supreme Court ruled on Monday that judges can’t impose mandatory life sentences without the possibility of parole on juveniles convicted of homicide.

Justice Elena Kagan announced the decision in Miller v. Alabama for a 5-4 majority, asserting that juveniles’ “lesser culpability” makes it unconstitutional for them to be sentenced under a scheme that does not give the judge the ability to consider factors such as the defendant’s age, maturity and upbringing in determining the sentence. “Youth matters,” Kagan said from the bench.

The ruling paves the way for roughly 2,000 inmates nationwide to seek reversal of their life sentences, according to Bryan Stevenson, executive director of the Equal Justice Initiative and the lawyer for the two inmates whose cases were ruled on by the high court.

Said Stevenson, “This is an incredibly important step forward in recognizing one of the great tragedies of our time,” namely the large-scale incarceration of juveniles under sentences that give them no hope of release after rehabilitation.

But Stevenson warned of a coming crisis in finding enough lawyers to represent the inmates who are entitled to relief because of Monday’s ruling. Many are in states that do not provide lawyers for indigent defendants on appeal.

The ruling marked the growing clout of Kagan in the Court, as well as the continuing importance of Justice Anthony Kennedy as the Court’s swing justice. He provided a fifth vote to the liberal wing justices Kagan, Ruth Bader Ginsburg, Sonia Sotomayor and Stephen Breyer. As the senior justice in the majority, Kennedy gave the job of writing the decision to Kagan, a major assignment for a relatively junior justice. Kagan joined the Court in 2010.

Reviewing the Court’s recent precedents, which have struck down the death penalty for juveniles and life without parole for non-homicide juvenile offenders, Kagan wrote, “imposition of a state’s most severe penalties on juvenile offenders cannot proceed as though they were not children.”

The decision came in the case of two 14-year-olds – Evan Miller and Kuntrell Jackson – who were convicted and sentenced with mandatory life in prison in Alabama and Arkansas, respectively. It stops short of totally prohibiting sentences of life without parole for juveniles, but bars them when judges have no other choice following a finding of guilt.

Justice Samuel Alito Jr. summarized his dissent from the bench with a tone of disgust. The majority had reached its result based on justices’ “personal views,” he said, rather than a careful assessment of what most state legislators have determined as penalties for murder. He ridiculed Kagan’s assertion that states have not explicitly authorized mandatory life without parole for juveniles, instead just allowing certain juveniles to be treated as adults – who are subject to such sentences. Legislators are “not that stupid,” said Alito. Justice Antonin Scalia joined Alito’s dissent.

Chief Justice John Roberts Jr. wrote the main dissent for himself, Scalia, Alito and Justice Clarence Thomas. The simple fact that more than 2,000 juveniles have been given mandatory life prisons without the possibility of parole, Roberts said, means that the punishment is “not unusual,” and therefore not unconstitutional under the Eighth Amendment.

The ruling won praise from defense lawyers and advocates for juveniles. “With today’s Supreme Court decision, America’s juvenile justice system became a little bit more humane and grounded in the scientifically demonstrable differences between juveniles and adults,” said Lisa Wayne, president of the National Association of Criminal Defense Lawyers.

The ruling was just one highlight of a news-filled day, the next to last session of the current term. The Court issued a major ruling on Arizona’s immigration law, and a per curiam decision rejecting a Montana Supreme Court ruling that ran counter to the high court’s 2010 decision in Citizens United v. FEC. Other action taken Monday:

• The Court granted review in 10 new cases for the fall term, including environmental cases testing the scope of the Clean Water Act. Decker v. Northwest Environmental Defense Center and Georgia-Pacific West v. Northwestern Environmental Defense Center involve a citizen suit to regulate storm water from logging roads and could affect the timber industry. “Going along with the arguments of radical environmentalists, the Ninth Circuit held that normal rainwater running off forest roads must be regulated as pollution under the Clean Water Act,” said Pacific Legal Foundation lawyer Daniel Himebaugh. “This astonishing concept would inundate the owners of forest roads with crippling new federal permitting requirements.” Justice Stephen Breyer recused, apparently because his brother Charles, a federal district court judge, sat by designation on the 9th Circuit panel that ruled on the cases below.

• The justices also added to their fall docket the case of Comcast Corp. v. Behrend, the next major dispute over class actions. At issue is whether class certification is permitted without resolving first whether class-wide damages could even be awarded.

• The Court denied review of a church-state dispute over the Mt. Soledad memorial in California, The U.S. Court of Appeals for the 9th Circuit ruled that the cross, situated on land taken by eminent domain by the United States, improperly conveys “a message of government endorsement of religion” that violates the Establishment Clause. Americans United for Separation of Church and State applauded the action. “Crosses belong in sacred spaces, not on public property,” said executive director Rev. Barry Lynn. “It’s time to move on and create a memorial that honors all of the men and women who made the ultimate sacrifice for this country.”

• After more than a year of deliberation, the justices denied review in Faculty Senate of Florida v. Florida, a challenge to a state law that banned the use of state funds for academic travel to Cuba.

Tony Mauro can be contacted at tmauro@alm.com.