The Supreme Court ruled yesterday that prosecutors may charge defendants with conspiracy even when government infiltration has made it impossible for the alleged conspiracy to achieve its goal, a decision that removed a lingering legal cloud over the use of sting operations against drug traffickers and terrorists.

With the assent of all nine justices, the court invalidated an interpretation of conspiracy law by the San Francisco-based U.S. Court of Appeals for the 9th Circuit. That court had said a conspiracy ends when the conspirators abort their plans or "the object of the conspiracy" is "defeat[ed]" because undercover law enforcement agents are already on the case.

"Where police have frustrated a conspiracy's specific objective but conspirators (unaware of that fact) have neither abandoned the conspiracy nor withdrawn . . . special conspiracy-related dangers remain," Justice Stephen G. Breyer wrote in the opinion for the court. "So too remains the essence of the conspiracy -- the agreement to commit the crime."

The 9th Circuit's rule, Breyer wrote, "would reach well beyond arguable police misbehavior, potentially threatening the use of properly run law enforcement sting operations."

The 9th Circuit was alone in this view among federal appeals courts, Breyer noted, so the Supreme Court's ruling brings the law in the nine western states covered by the 9th Circuit into line with that of the rest of the country.

At a time of heightened concern over terrorist plotting within the United States, federal law enforcement officials say their efforts hinge on infiltrating cells and frustrating attacks before they occur. Conspiracy laws, always a popular tool of prosecutors, are considered especially useful in taking on terrorists.

The Bush administration made this argument in its brief, noting that the 9th Circuit's decision "would seriously compromise the effective investigation and prosecution of conspiracies, not only in drug cases, but in terrorism and other criminal contexts."

The case, U.S. v. Jimenez Recio, No. 01-1184, began in November 1997, when police stopped a truck loaded with illegal drugs in Nevada. Government agents persuaded the drivers to cooperate with them in a sting operation, whereby they continued to the truck's intended destination in Idaho, where two traffickers met the truck and were promptly arrested.

The two traffickers, Francisco Jimenez Recio and Adrian Lopez-Meza, were convicted in federal court of conspiracy to sell illegal drugs, but appealed, arguing that there was insufficient evidence to prove that they were involved in such a conspiracy because it had already been effectively thwarted.

In a 2001 ruling, the 9th Circuit agreed, and the federal government asked the Supreme Court to reverse the ruling.

Attorneys for Jimenez Recio and Lopez-Meza argued that the 9th Circuit had not articulated a new rule of law but had merely decided that, on the facts of their case, the alleged conspiracy was over.

Justice John Paul Stevens dissented in part yesterday, agreeing with the other eight justices' analysis of conspiracy law but noting that the government should not have been allowed to appeal to the Supreme Court for procedural reasons.

Separately, the court announced yesterday that oral arguments in the University of Michigan affirmative action cases will take place on Tuesday, April 1.

The justices will hear one hour of argument each on Grutter v. Bollinger, No. 02-241, a challenge by white students to Michigan's law school admissions program, and on Gratz v. Bollinger, No. 02-516, a challenge by white students to Michigan's undergraduate admissions program.