Forget beans and bullets. Modern armies run on batteries and barrels of oil. The average U.S. soldier consumes 88 AA batteries during a five-day mission. Sights for thermal weapons, GPS receivers, and night-vision equipment require a lot of juice. Early in the Iraq war, the U.S. Army burned through 100,000 large-volume lithium-sulfur dioxide batteries, which power everything from radios to anti-tank missile launchers, each month. And when soldiers aren’t changing batteries, they’re filling the gas tanks of Humvees, Abrams tanks, and armored personnel carriers. The U.S. military guzzles some 2.4 million gallons of fuel every day in Iraq and Afghanistan; around two thirds of the gross tonnage that soldiers cart around in combat is fuel.

There is a high cost, in both lives and treasure, of getting energy to the battlefield. So, the Pentagon is pushing for renewable energies with an urgency that would make even Al Gore smile. The Army, for instance, plans to soon field the Transportable Hybrid Electric Power Station, a mobile generator that combines solar panels, a wind turbine, a diesel generator, and storage batteries. The most notable push for green power occurred last year, when Maj. Gen. Richard Zilmer, a top U.S. Marine commander in Iraq, put in a “Priority 1” request (the most urgent) for solar panels and wind turbines. Why? Reducing the Marines’ dependence on fossil fuels, Zilmer argued, would save lives. Every gallon of fuel delivered to his forward operating bases had to be trucked in via vulnerable ground convoys. In fact, when all of the costs are factored in—storage, transportation, and security—getting just a single gallon of fuel delivered to the battlefield costs hundreds of dollars.

That has the Pentagon taking a hard look at much of its battlefield equipment, too. Among other things, the armed services are...