When first lady Michelle Obama kicked off her federal campaign against childhood obesity Feb. 9, she declared that “this epidemic … impacts the nation’s security.” So you had to wonder: Would the administration fight the War on Fat with a law enforcement approach, or would it get down to business and start declaring chubby kids “enemy combatants”?

Actually, as nanny-state campaigns go, the first lady’s isn’t terribly militant. The administration wants tax incentives for grocery stores in underserved neighborhoods, revised food labeling by the FDA, more money to encourage schools to serve healthier food and, of course, more “public service” hectoring of parents and kids.

The campaign is mostly harmless, but it’s unlikely to do much good. If the feds want to make Americans thinner, they’d do better to get out of the way and leave us alone.

History suggests that the federal bully pulpit isn’t very effective when it comes to getting junior to put down his Twinkies. As Matt Labash wrote in his entertaining new book, “Fly Fishing with Darth Vader,” when President John Kennedy jumped into the fight against fat in 1962, he used “language that today would see him impeached,” railing against “soft, chubby, fat-looking children” who sit on the sidelines while their classmates play basketball.

Seeking to instill “vigah” into the little porkers, JFK published an article in Sports Illustrated warning, à la Michelle, that “the Soft American” was “a menace to our security.” The only solution was to make “the physical fitness of our youth … a direct responsibility” of the federal government.

Other presidents continued Kennedy’s campaign, with George H.W. Bush appointing Arnold Schwarzenegger to head the President’s Council on Physical Fitness. All the while, kids got fatter, with the percentage of overweight children more than doubling in the past 30 years. (Come to think of it, these days the Governator looks pretty flabby himself.)

Instead of nagging parents and private companies, the feds ought to consider how their own policies have contributed to Americans’ expanding girth.

Science journalist Gary Taubes said federal advocacy for a low-fat diet, starting in the ’70s, helped shift Americans toward a diet emphasizing the carb-heavy bottom of the USDA food pyramid. “We ate more fat-free carbohydrates,” Taubes wrote, “which, in turn, made us hungrier and then heavier.”

Farm subsidies were a major contributing factor. Take the Twinkie, food journalist Michael Pollan wrote, it’s “basically a clever arrangement of carbohydrates and fats teased out of corn, soybeans and wheat — three of the five commodity crops that the farm bill supports.”

And anyone who, like the first lady, supports healthy, locally grown foods ought to consider how the morass of federal regulations and subsidies privilege large corporate agribusiness over small farms.

At the end of the day, if America’s kids are getting fatter, it’s mostly their parents’ fault. Too many, for too long, have paid too little attention to what their kids are eating.

The federal government can’t solve that problem. But it can certainly stop making a parent’s job harder.