The Supreme Court’s four more-liberal members voted to allow federal prosecution of medical-marijuana users — including cancer patients who grow small quantities at home to alleviate agonizing pain — even in the 11 states that have legalized medical marijuana. So did centrist Justice Anthony Kennedy and conservative Justice Antonin Scalia.

The three dissenters were the other two conservatives, Chief Justice William Rehnquist and Justice Clarence Thomas, and the other centrist, Sandra Day O’Connor. They said that the federal government lacked the power to apply the sweeping federal anti-drug law to the use of homegrown plants under the laws in California and 10 other states allowing physician-recommended use of marijuana to ease symptoms of illness.

Why are the liberals siding with the Bush administration’s drug warriors against patients desperate for relief? Have the conservative Rehnquist and Thomas gone soft on drugs? And what does this 6-3 decision, on June 6, in Gonzales v. Raich really mean?

It does not mean that federal prosecution of medical-marijuana users will immediately step up; other legal challenges are pending. It does mean five things, at least:

• The case was not mainly about medical marijuana. It was about the breadth of congressional power to override state law, and about states’ rights.

• The decision shows that the Rehnquist Court’s "states’-rights revolution," so feared by liberals and hailed by (some) conservatives, will never be a major impediment to federal regulatory power. Not, that is, unless at least four new appointees join Thomas’s solitary push to strike down dozens of federal laws and overrule dozens of precedents going back more than a century — a most unlikely scenario.

The feverish predictions of a states’-rights revolution began with the Court’s 1995 decision in United States v. Lopez — the first since 1935 striking down an act of Congress as exceeding its power to regulate interstate commerce. The Court’s conservatives and centrists ruled by 5-4 that Congress had usurped the powers of the states when it banned possession of guns within 1,000 feet of a school. Fear and loathing coursed through liberal law faculties.

But a careful reading of Lopez never supported such fears. Rehnquist’s opinion did not question the Court’s many precedents expanding the scope of congressional power under the commerce clause. Rather, he stressed that none had gone so far as to support a congressional ban on a purely intrastate activity involving neither commerce nor an economic transaction. To uphold the guns-near-schools law would have been to extinguish all limits on what James Madison called the "few and defined" powers delegated to the federal government. Kennedy and O’Connor stressed, in a concurrence, that the decision was "limited" and that they had no intention of upsetting "the stability of our commerce-clause jurisprudence."

Decisions over the past few years have tended to confirm this, with both Rehnquist and O’Connor moving to limit the conservative push to protect states’ sovereign immunity from damage lawsuits. Now Scalia’s concurrence upholding congressional power to ban medical marijuana shows that he has a fairly broad view of the commerce clause. And even Rehnquist, who has championed states’ rights for more than 30 years, has never called for overruling the major commerce-clause precedents.

Indeed, of the 34 justices appointed since 1937, when the Court began upholding major New Deal programs, only Thomas has called for a truly revolutionary cutback in Congress’s commerce power and radical expansion of states’ rights.

• The Rehnquist-Scalia-Thomas "conservative bloc" is hardly the monolithic alliance that it is sometimes portrayed to be. To be sure, the three are consistently allied (usually in dissent) on hot-button issues like abortion, gay rights, and racial affirmative-action preferences. But they split three ways in the medical-marijuana case: Scalia was less supportive of states’ rights than Rehnquist (who joined O’Connor’s dissent), and Rehnquist was less supportive than Thomas (who dissented more broadly).

• If President Bush hopes to push the Court dramatically to the right, the odds are against his having more than modest success. The medical-marijuana decision makes it clear that a real states’-rights revolution could occur only if at least three justices besides Rehnquist were to retire, and only if all four of their successors were to ally with the so-far-solitary Thomas. And it’s hard to believe that Bush himself wants a radical cutback in federal regulatory power. That would render unconstitutional much of his own legislative agenda on Social Security, Medicare, and more.

The Court is much closer to a tipping point on social issues such as abortion, gay rights, racial preferences, and religion. But replacing the conservative, 80-year-old Rehnquist with another conservative would leave the balance unchanged on those issues. And Roe v. Wade would be at risk only if two of its six supporters were to step down and if both of their replacements (as well as Rehnquist’s) were to vote to overrule what even many conservatives consider a settled precedent. That’s possible, but not likely, at least on Bush’s watch.

• For all the attacks on the justices, this case is a reminder of the indispensability of a strong Supreme Court and the value of an ideologically diverse one.

Although we have come to take for granted the justices’ power to settle the kind of state-federal dispute at the heart of the medical-marijuana case, a weak Court would have trouble making such decisions stick. And "in most matters, it is more important that the … law be settled than that it be settled right," in the words of Justice Louis Brandeis.

As for ideological diversity, this decision illustrates the vast room for reasonable disagreement about how to apply our 216-year-old Constitution to today’s complex world. Each of the four separate opinions makes powerful constitutional arguments. None seems an effort to legislate the justices’ personal policy preferences. (Would that it were ever so!)

Indeed, the majority opinion, by Justice John Paul Stevens, implies in a footnote that perhaps Congress should legalize medical marijuana — but holds that until Congress does, the Court must defer. Conversely, Justice O’Connor’s dissent asserts, "If I were a California citizen, I would not have voted for the medical-marijuana ballot initiative" — but holds that the Court and Congress must respect California’s right to experiment.

Was the decision correct? No and yes. As a policy matter, Congress as well as the states should legalize medical marijuana, with strict regulatory controls. The proven benefits to some suffering patients outweigh the potential costs of marijuana being diverted to illicit uses.

But the issue for the Court was not whether medical marijuana should be legal. It was whether Congress or the state has the ultimate power to decide. And while that is a very close call, the Stevens majority opinion and the Scalia concurrence had a bit the better of the argument, in my tentative view.

All nine justices agreed that to effectuate Congress’s undoubted power to regulate (or ban) interstate commerce in marijuana, Congress can ban intrastate commerce and possession as well. The disagreement was about whether Congress can extend this ban to noncommercial cultivation and medical use, even where legal under a state medical-marijuana law.

The majority stressed that supposedly "medical" marijuana could easily be diverted into interstate commerce, thus circumventing the comprehensive federal effort to curtail such commerce. The dissenters responded that states should be given room to experiment, and that neither Congress nor the Bush administration has produced much evidence of medical marijuana being diverted to the interstate market. The majority rejoined that the Court must defer to Congress’s concern on this score, and that if states could exempt homegrown medical marijuana from Congress’s ban, the same logic would also allow states (hypothetically) to exempt homegrown recreational marijuana. And so on, down the slippery slope.

Such constitutional ratiocinations seem beside the point to suffering patients such as Angel Raich, who told reporters that only marijuana enables her to eat and that she will continue using it because "if I stopped, I would die." Thousands of others who suffer from AIDS, cancer, and other severe illnesses can relieve their agony only by using marijuana.

So as a matter of constitutional power, the Court may have been right to defer to Congress. But as a matter of common decency, Congress and the administration are wrong. They should stop carrying their drug-war obsession to the point of denying the most effective source of relief to people who live in excruciating pain.