The U.S. Supreme Court last week upheld a trial court's decision ordering the state of California to reduce its prison population by at least 40,000 inmates over the next two years.

The state prison system has been filled to about double its design capacity over the past decade. The physical crowding was deplorable. But what gave rise to the finding of cruel and unusual punishment in the case of">Brown v. Plata was the prison system's chronic failure to provide basic medical and mental health treatment facilities. The trial determined that those failings could not be remedied without significant reductions in inmate population.

Justice Anthony Kennedy, writing for a 5-4 majority of the high court, affirmed the trial court's decision to cap the prison population at 137.5 percent of capacity. That means a California population that stood at 156,000 at the time of trial will have to be cut to about 110,000 inmates over the next two years.

The case has important implications for corrections policy in Missouri and Illinois. Missouri's prison population is steadily maintained at just below the system's capacity of about 31,000.

In Illinois, however, the prison population is expected to push 50,000 by later this year, to about 133 percent of capacity.

What makes the Brown case interesting is how dissenting justices Antonin Scalia and Samuel Alito characterized the federal court intervention in California.

Mr. Alito noted that the state may be forced to release "the equivalent of three Army divisions" of criminal convicts. Mr. Scalia ominously predicted that "terrible things are sure to happen as a consequence."

To both justices we say, "Welcome to Missouri."

Missouri has been doing on the installment plan what California is being pushed to do by federal court order. Illinois may not be far behind.

Missouri's incarceration rate (509 per 100,000 population) is higher than California's (458 per 100,000), and significantly higher than Illinois' (349 per 100,000).

Missouri prisons had 18,672 new admissions in FY 2010. They were accommodated without causing massive crowding by releasing an equal number inmates.

In just that one year, to use Mr. Alito's parlance, the equivalent of more than an Army division was released from Missouri prisons to probation and parole.

Some of them will go on to commit new crimes. "Terrible things are sure to happen."

That's why it is vitally important to ensure that prison systems are reserved for dangerous offenders — and that alternatives to incarceration be used to deal with nonviolent lawbreakers.

That means making careful, well-informed choices at the time of sentencing. It requires keeping and reporting">detailed data.

California is about to embark on a crash course.

Missouri Supreme Court Chief Justice William Ray Price Jr.">calls it being 'smart on crime." There's a growing political consensus that swollen penitentiaries have not made Missourians safer.

The financial savings could be immense. The average cost of community supervision for probation and parole in Missouri is $3.92 a day — a sum that rises to $10.18 when electronic monitoring is included. The per diem for prison is $42 to $50.

Sentencing reform means being honest with the public. In every state prison system — not just those subject to federal court oversight — inmates are being moved through a revolving door every day. Smart states know which inmates are on which side of the door.