Thousands of citizens in Baltimore and other American cities have died in recent years in an epidemic of gun violence. The contagion is carried by a flood of weapons, legal and illegal, that presents a frustrating challenge to police, prosecutors and politicians attempting to calm the cities. Yesterday, the conservative majority of the U.S. Supreme Court significantly complicated that effort to control violence with a 5-4 decision that struck down a Washington, D.C., law that bans private ownership of handguns in that city. Predictably, the National Rifle Association immediately announced plans to file lawsuits aimed at doing away with more-limited gun control laws elsewhere, and advocates of tight gun controls were rightly dismayed. While the court made it impossible to outlaw handgun ownership, it recognized that laws setting reasonable limits on the ownership and use are appropriate. And now advocates of gun control should use the court's words to make a strong case for tough new laws aimed at keeping guns out of the hands of criminals and away from schools and other public places and to require tighter registration to better track handguns and make owners more accountable for crimes committed with their weapons. These new laws should be much more difficult to oppose now that gun control opponents have been deprived of one of their principle arguments against stronger gun rules - that any gun control proposal represents the first step toward an outright ban. The Supreme Court has said definitively that handguns can't be banned. Now members of Congress who have pandered to the NRA or knuckled under to its threats should recognize the damage that illegal handguns are doing and support legislation to reduce the illegal weapons that travel from state to state. Much more effective federal handgun sales regulation is needed, including crackdowns on gun dealers who sell many weapons to shadow purchasers who pass them along to illegal users. This spring, Baltimore joined New York and other cities in a campaign to stop the interstate flow of illegal guns. It may take years for the courts to determine just how much handgun control is enough, but the evidence that tougher limits and penalties are urgently needed can be found daily in newspaper crime reports across America. More than 29,000 people were killed by firearms in 2004, the most recent year for which statistics are available. Another 64,000 were injured - more than 170 a day. Baltimore knows firsthand the agony of that slaughter.