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WASHINGTON — An army marches on its stomach, according to an old military adage.

Today’s military is tethered to a gas pump.

The amount of fuel used per individual soldier has skyrocketed in recent years because of an increased use of aircraft and armored vehicles. In Afghanistan, that dependency has meant long and costly supply lines that are vulnerable to attack and limit the reach of American forces.

The Pentagon increasingly sees this energy dependence as a military weakness and is trying to reduce it. The Navy is attempting to transition to biofuels for its ships and planes, and the Army and Marine Corps are exploring a host of initiatives, including using solar energy to power radio batteries.

“Every time some yahoo says ‘I’m going to close the Strait of Hormuz’ (the price of) oil spikes,” Navy Secretary Ray Mabus told USA TODAY in an interview.

In the past, Iran has threatened to close the Strait of Hormuz, through which 20% of the world’s oil supply travels.

“Right now, we buy our oil from foreign sources, and some of those sources don’t have our best interest at heart,” Mabus said.

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SOLDIER SOLAR PACKS

The services are also developing portable combat outposts powered by fuel-efficient generators and solar panels. The Marine Corps is experimenting with small, flexible solar panels that can be attached to a Marine’s uniform.

“We’ve looked at everything,” said Col. Bob Charette, director of the Marine Corps’ expeditionary energy office. He said solar was the most mature of the industries.

Modern communications technology and precision weapons have increased the lethality of the Marine Corps and allowed commanders to disperse their units at ever greater distances, Charette said.

But it has came at a price: a dangerous dependence on fuel.

“Because of our thirst for liquid fuel, we’re not as light and agile as we once were, putting both our Marines and our expeditionary capabilities at risk,” Gen. James Amos, the Marine Corps commandant, said bluntly when he launched a new energy strategy in 2011.

In 2001, a Marine infantry battalion, which typically has about 800 men, had 64 Humvees. Ten years later, that same battalion has 173 armored vehicles, which are each between 3,000 and 5,000 pounds heavier than Humvees, according to a Marine Corps study.

The increased reliance on technology has also driven the demand for fuel. The Marine Corps had 8,000 laptops in Helmand province, the rugged region in southwest Afghanistan. At its peak, there were about 21,000 Marines in Helmand.

The intensifying thirst for fuel is not limited to the Marines. Since the Vietnam War, there has been a 175% increase in the demand for fuel per servicemember. Today, the Defense Department spends about $15 billion a year on fuel, and 60% of it comes from foreign sources, according to the Pentagon.

AFGHANISTAN’S ENERGY DRAIN

In Afghanistan, America’s massive war effort has been supplied by civilian convoys that have to move through two major supply lines — one through Central Asia in the north and the other through Pakistan in the east.

The power needs in Afghanistan, a landlocked country divided by steep mountains, are vast. Bagram, the sprawling American air base 30 miles from Kabul, uses about 50 megawatts of power, equivalent to a small American city. The military moves about 40 million gallons of fuel a month into Afghanistan to power generators and fuel aircraft and vehicles.

Much of the fuel moving into Afghanistan is Russian, says Sharon Burke, the assistant secretary of Defense for energy issues. On the wall of her Pentagon office is a large map of Afghanistan marked with supply lines, a reminder of the vulnerability of those chokepoints through which the supplies move.

“You put a supply line like that in a battlefield, and it’s going to be a limiting factor,” Burke says.

Once the fuel gets to Afghanistan, it has to roll across narrow dirt tracks or get flown into remote combat outposts. That can be deadly for the soldiers and Marines who take the fuel to those sites.

The Pentagon insists its initiatives to develop sustainable energy sources are motivated only by military necessity.

“It’s not about being environmental,” says John Conger, a Pentagon official who oversees the department’s installations and energy requirements. “The environmental stuff is a co-benefit.”

Lawmakers are mostly supportive of efforts to reduce the military’s dependence on fuel, but some critics in Congress say the Navy’s more ambitious plan to transition to biofuels has gone beyond those limited military objectives and is a thinly disguised environmental initiative.

“It’s the secretary of the Navy’s green agenda,” says Rep. Randy Forbes, a Virginia Republican. Forbes said the expenditures on biofuels has meant less money for shipbuilding and operation and maintenance of the Navy’s fleet. “He never stopped to say, ‘What’s the price tag?’ ” Forbes said of Mabus.

A centerpiece of Mabus’ initiative was the Great Green Fleet, a demonstration last year of the Navy’s ability to operate its ships and aircraft on biofuels.

During the demonstration, the Navy powered a carrier strike group, which consists of escort ships and aircraft, with 50% biofuels over a two-day exercise. The biofuels were made from a number of sources, including used cooking oil and algae.

The Green Fleet name is a reference to President Theodore Roosevelt’s Great White Fleet, a battle group that circled the globe in a demonstration of American seapower in the early part of last century.

Critics saw the Great Green Fleet as a demonstration of wasted taxpayer money. Sen. James Inhofe, R-Okla., has pointed out that the Navy spent $12 million for biofuels at $27 a gallon for the demonstration.

The Navy acknowledges it has paid a premium for biofuels, but insists it is for experimental and test purposes only until the price becomes competitive with conventional fuel.

The Navy’s use of the Defense Production Act, designed to allow the military to support industries considered critical to national security, to invest in biofuel refineries has drawn criticism.

“The Navy wants to spend hundreds of millions of dollars in companies to create the market,” Forbes says.

Mabus sees the investment in biofuels as a hedge against the vagaries of the world oil markets.

U.S. military aircraft and ships patrolling the Persian Gulf or in the far reaches of the Pacific are forced to purchase much of their fuel from foreign suppliers, where they are hostage to price fluctuations and vulnerable to supply disruptions by rogue states.

Mabus said the Navy has faced skeptics before.

“The Navy has always been on the forefront of changing energy use,” Mabus says. He said there were skeptics when the Navy moved from wind to steam.

“Every single time, those naysayers were absolutely wrong,” Mabus says. “If price had been the only consideration, we’d still be using sails.”