U.S. Navy officers and sailors stand at attention as U.S. Navy ship USS Freedom (LCS 1) berths at the Changi Naval Base on Thursday April 18, 2013 in Singapore. (AP Photo/Wong Maye-E)

When Congress returns from vacation in a week and a half, the Senate will take up the National Defense Authorization Act (NDAA), the bill that sets the Department of Defense (DoD) budget — and defense-related budgets for other departments — and mandates how America’s military leaders use that funding.

Last year, the fight over the NDAA got stuck on the Pentagon’s plans to make the military’s operations greener and more sustainable. Republicans in particular went after Navy Secretary Ray Mabus’ plan for a “great green fleet” powered on alternative energy — a play on Theodore Roosevelt’s vision of a “great white fleet” that would circumnavigate the world and define the U.S. as a key international player. Mabus’ goal is for his forces to draw 50 percent of their energy from alternative sources by 2020.

The Department of Defense, with energy costs topping $4 billion each year, set slightly lower — but still notable — sustainability goals for the rest of the military. DoD will “deploy three gigawatts of renewable energy – including solar, wind, biomass and geothermal – on Army, Navy and Air Force installations by 2025 – enough to power 750,000 homes,” the White House announced last year.

Some legislators balked when reports surfaced that the Navy would use biofuels that might cost four times more than the petroleum they’re using now — or, by some iffy calculations, seven times more. In response to an article published by Wired’s Danger Room blog stating that “the Navy could spend as much as $1.76 billion annually for all the biofuel they’ve promised to use by 2020,” Deputy Assistant Secretary of the Navy for Energy Tom Hicks wrote an op-ed rebuttal saying that the math didn’t check out.

We just don’t live in a world where oil prices never go up, technology doesn’t advance, and economies of scale don’t bring down cost; in fact, history tells us that the exact opposite is true. Eight years ago, the cost of petroleum was just under $40 per barrel and the annual volatility was plus or minus 10 percent. Today, the price of petroleum has more than doubled and the annual volatility is more than 30 percent. It is impossible to accurately predict where prices will be eight years from now, but with ever-increasing global demand and continued political unrest in oil-producing countries, nearly all experts agree that oil prices will increase, and we have seen the price of biofuel drop.

Hicks’ argument drew on a larger, tactical plan for a more sustainable military that strategists have been advancing for years. Writing in The Hill, Retired Marine Corps Lt. General John Castellaw argues that “the military, like all consumers in the U.S., relies on oil from countries that do not align with our interests. This affects our foreign policy and undermines our national security.”

The United States military is the single largest purchaser of petroleum fuel in the world, burning through about 325,000 barrels of fuel per day. Almost all of that fuel is derived from oil. This is important not because of the vast carbon footprint (or boot print) that the military has – a separate, and important problem. It is actually the dependence on oil that presents the military with a long-term strategic risk.

At a demonstration of biofuel-powered helicopters and fighter jets last summer, Mabus explained that the Navy paid a billion dollars in some months for energy because of rising oil prices — and much of that oil was sourced from foreign countries. “We simply have to figure out a way to get American made homegrown fuel that is stable in price, that is competitive with oil that we can use to compete with oil. If we don’t we’re still too vulnerable,” he said.

The military has also made it clear that their sustainability programs have another potential strategic benefit: Blunting climate change is of grave concern to America’s national security community. For the last few years, the Pentagon has put significant resources into examining the likely effects of climate change on global stability.

“You have the real potential here in the not-too-distant future of nations displaced by rising sea level,” Navy Admiral Samuel J. Locklear III, the chief of U.S. forces in the Pacific, told the Boston Globe earlier this year. “Certainly weather patterns are more severe than they have been in the past. We are on super typhoon 27 or 28 this year in the Western Pacific. The average is about 17.”

These kinds of global shifts from a changing climate, Locklear said, “will cripple the security environment, probably more likely than the other scenarios we all often talk about.’’ (This coming from a commander whose job includes monitoring the nuclear ambitions of North Korea and cyber-attacks from China.)

A last-minute Senate vote last November allowed the DoD’s sustainability programs, including Mabus’ ambitious plan for the Navy, to proceed. But this year, once again, the initiatives will face criticism similar to what they encountered last fall. A version of the NDAA that has already made it through the House would prevent the Navy from purchasing biofuels that are more expensive than oil until the sequester is lifted.

For years, the military has been an agent of change for the American economy, capable of prompting technological revolutions worldwide. The military industrial complex launched the semiconductor industry and played a lead role in the creation of the Internet. Mobilization for war helped pull the country out of the Great Depression. General Castellaw writes in The Hill that “from the age of sail to steam to oil to nuclear, the United States Navy in particular has a history of swiftly transitioning to new energy sources when a strategic need arises. Today is no different: the dependence on oil for all flight operations and most sea transportation presents the Navy with long-term strategic risk.” If the Navy and the military on the whole are allowed by Congress to meet their sustainability goals, they could lead the way for the rest of the world.