But there was almost no indication for decades that employer-sponsored health insurance would spread from sea to shining sea.

World War II disrupted those trends. As demand for everything — particularly labor — climbed, Congress passed the Stabilization Act of 1942, which allowed the president to freeze wages and salaries for all the nation’s workers. A day after its passage, President Franklin Roosevelt issued an executive order invoking these powers, which applied to “all forms of direct or indirect remuneration to an employee,” including but not limited to salaries and wages, as well as “bonuses, additional compensation, gifts, commissions, fees.”

But there was an exemption of massive proportions slipped into a fateful clause: “insurance and pension benefits” could grow “in a reasonable amount” during the freeze.

As companies struggled to deal with wartime labor shortages, the wage freeze left them in a serious bind: How could they retain workers if they couldn’t give raises? If they didn’t soon realize the allure of fringe benefits, insurance companies pressed that case through marketing campaigns, as historian Jennifer Klein has observed.

The Revenue Act of 1942 triggered another rush to enroll employees in health plans. By slapping corporations with tax rates of 80 or even up to 90 percent on any profits in excess of prewar revenue, Congress all but guaranteed a frenzied search for loopholes. Employee benefits, according to the new law, could be deducted from profits. As an anonymous employer observed in a study published on trends in health insurance, “it was a case of paying the money for insurance for their employees or to Uncle Sam in taxes.”