Compulsion is as essential to government as water is to snow. Most of what governments do requires forcing people to do things they might choose not to -- pay taxes, stop at red lights, respect the property of others, avoid creating public nuisances, and so on. But the measure of a free society is its willingness to use compulsion only when it is indispensable. And by that measure, the state of Texas is falling short.

Recently, Gov. Rick Perry issued an executive order that surprised both friends and foes -- stipulating that before being admitted to sixth grade, every girl must be vaccinated against the human papillomavirus (HPV), a common sexually transmitted disease that causes genital warts and cervical cancer. In a reversal of their usual view of the conservative Republican Perry, Planned Parenthood of North Texas applauds the measure, while Concerned Women for America opposes it.

Mandatory immunization, far from being a new development, goes back to the 19th century. But these days, it's generally imposed on schoolchildren only for serious illnesses that can spread easily by casual interaction, such as measles, whooping cough, diphtheria and polio. HPV is not one of those.

There are sound reasons to immunize pre-adolescents. One is that it makes sense to confer protection well before potential victims become sexually active -- by which time it may be too late. In a startling revelation, the Food and Drug Administration reports that "at least 50 percent of people who have had sex will have HPV at some time in their lives." Even someone who is a virgin upon marrying and monogamous afterward can be unwittingly exposed to it by a spouse.

Nor is immunization useful only for girls who will choose to have sex -- since they may be molested by someone carrying the virus. And a clinical study that led to FDA approval found the vaccine is nearly foolproof in preventing two types of HPV that account for 70 percent of all cervical cancers, which kill nearly 4,000 American women each year.

But the inoculation (which has not yet been approved for boys) is new, and it addresses a disease that most people know nothing about. A mandate would force on people something that most of them, in time, probably can be persuaded to accept voluntarily. The first need is public education, in the form of a comprehensive effort to inform Americans and their doctors of the dangers of HPV and the efficacy of immunization.

Even medical experts are wary of forcing it on the unwilling. The Texas Medical Association, which says many of its members are already administering the vaccine, does not favor requiring it. The American Academy of Pediatrics (AAP), which recommends the vaccine, has also declined to endorse the mandate.

"We don't know if it will be necessary," says Joseph Bocchini, a Louisiana pediatrician who heads the AAP's committee on infectious diseases. Given sufficient information about this new protection, he believes, "most people would want it for their daughters." But making it a requirement for school admission would risk a backlash against the vaccine before it has even gotten a foothold. It might also prod more parents to reject other immunizations.

Texas law, after all, allows parents to exempt their children from any required vaccine. Currently, less than 2 percent of kids in the state opt out, but that number will undoubtedly rise once HPV is in the mix. Groups that see the inoculation as potentially dangerous, or as an implied approval of premarital sex, will undoubtedly mobilize to encourage mass refusal. Parents who would be inclined to accept something recommended by the family pediatrician may resist if it's commanded by the state.

The consensual approach avoids those hazards, and it can be highly effective. The federal Centers for Disease Control and Prevention report that, though they are not covered by school vaccination requirements, more than 90 percent of children under age 3 are immunized against polio, measles, mumps, rubella and hepatitis B.

The HPV vaccine ought to get a chance to establish itself on a voluntary basis before any state makes it mandatory. Public health is one of those areas in which, even by my libertarian lights, the government is sometimes entitled to force people to take measures to protect themselves and others. But compulsion should be a last resort, not a first.