Mayor Bloomberg wants to take New York City’s gun control regulations nationwide. At his swearing in ceremony earlier this month, Mr. Bloomberg announced his top priority for the next four years: a nationwide fight across America for more gun control, from Washington, D.C., to individual statehouses.

The current push for more gun control stems from the tragic murders of two New York City police officers last year, following in the wake of two officers killed in 2003 and 2004. Mr. Bloomberg has long supported every gun regulation possible, even banning off-duty or retired police officers carrying guns near city hall. He is already pushing for tougher gun control in New York state, claiming that otherwise law-abiding New York gun-owners – who already pass all the local, state, and federal gun control regulations – are an important way his city’s criminals obtain guns. Those same motivations are behind the program that he now wants to take nationwide.

Everyone wants to prevent criminals from getting guns. But the experience in other countries, even island nations that have gone so far as banning guns and where borders are relatively easy to monitor, should give Mr. Bloomberg and his supporters some pause. The regulations seem to have only kept law-abiding citizens from getting guns.

Not only didn’t violent crime and homicide decline as promised, but they actually increased.

* The British government banned handguns in January 1997 but recently reported that gun crime in England and Wales nearly doubled in the seven years from 1996 to 2003. Since 1996, the rate of serious violent crime has soared by 88%, armed robberies by 101%, rapes by 105% and homicide by 24%.

* Australia’s 1996 gun-control regulations banned many types of guns and the immediate aftermath was similar. While murder rates remained unchanged, armed robbery rates averaged 59% higher in the eight years after the law was passed (from 1997 to 2004) than in 1995.

* The Republic of Ireland banned and confiscated all handguns and all center-fire rifles in 1972, but murder rates rose five-fold by 1974 and in the 20 years after the ban has averaged 114% higher than the pre-ban rate (never falling below at least 31% higher).

* Jamaica banned all guns in 1974, but murder rates almost doubled from 11.5 per 100,000 in 1973 to 19.5 in 1977, and soared further to 41.7 in 1980.

The two police officer murders last year in New York City had something else in common: The murders involved drugs.

Drug gangs have a lot at stake and they can’t simply call up the police when another gang encroaches on their turf, so they end up essentially setting up their own armies. (Nor can drug users call the police when someone steals their cache.) Just as gangs find ways to smuggle drugs in from Latin America and Asia, they will also find ways to smuggle in weapons to defend their turf.

Letting more law-abiding citizens own guns may actually save police lives. There are also a large number of peer-reviewed academic studies showing that letting private citizens own guns reduces violent crime, and some work finds that gun crime falls even faster than overall violent crime. Others have directly linked this reduction in crime to officer safety. Professor David Mustard in the Journal of Law and Economics specifically tested this and found that on average each additional year a state allows citizens to carry concealed handguns reduces the number of police murders by another 2%.

Even for politicians, hard facts must eventually matter. If they can’t see that gun control laws have failed to deliver as promised, it’s hard to know when facts will make a difference.

Mr. Lott, a resident scholar at the American Enterprise Institute, is the author of “More Guns, Less Crime” (University of Chicago Press, 2000).