In promoting the use of renewable energy, U.S. administrations have put considerable focus on the U.S. military. That’s a logical priority: The U.S. Department of Defense is the largest single consumer of energy in the United States, and it consumes close to 80 percent of the federal government’s total. In 2009, Obama administration Navy Secretary Ray Mabus began planning for half of the U.S. Navy’s energy use to be supplied by non-fossil fuel sources by 2020—a year that came and went—and for half of the Navy’s onshore installations to have net-zero carbon emissions.

In promoting the use of renewable energy, U.S. administrations have put considerable focus on the U.S. military. That’s a logical priority: The U.S. Department of Defense is the largest single consumer of energy in the United States, and it consumes close to 80 percent of the federal government’s total. In 2009, Obama administration Navy Secretary Ray Mabus began planning for half of the U.S. Navy’s energy use to be supplied by non-fossil fuel sources by 2020—a year that came and went—and for half of the Navy’s onshore installations to have net-zero carbon emissions.

After the Trump administration abandoned these plans, U.S. President Joe Biden appointed six senior officials to the Defense Department to advance climate policy in the U.S. military. Biden’s senior climate appointee at the Pentagon, Joe Bryan, advocated for canceling the military’s national security exemption for emissions reduction. In the 2022 U.S. National Security Strategy, there are more mentions of “climate” than of “China” or “Iran,” and the single mention of “energy security” focuses on transitioning from fossil fuels. The Biden administration has also promoted greater electrification of the battlespace in order to increase the use of renewable energy.

There is little sign of progress toward any of these goals. Petroleum-derived fuels continue to supply essentially all of the U.S. military’s operational needs, with the exception of nuclear-powered submarines and aircraft carriers. On noncombat military installations—responsible for about one-third of total Defense Department energy use—natural gas and electricity provide most of the energy. Most of the electricity, in turn, is also still derived from fossil fuels.

Militaries will remain large consumers of fossil fuels for many decades to come, simply because today’s dominant forms of renewable energy—wind and solar power—are poor substitutes that don’t meet the operational needs of armed forces. Militaries depend on fossil fuels due to five factors: their need for energy-dense fuels; new weapons and platforms that require more energy than those they replace; the lack of tolerance for energy disruptions; the lack of flexibility in weapons and platform design to change fuel sources; and the infeasibility of electrification of the battlespace.

First, the military needs fuels that pack a lot of energy per unit of weight and mass. Not only does the fuel have to generate enough power to propel a tank, plane, or naval vessel over substantial distances, it also must be easily transportable in a constrained space. Today, only petroleum-derived and nuclear fuels can provide the needed energy density. Batteries, for instance, pack less energy per pound and inch than these fuels. Biofuels are also less energy dense than petroleum derivatives.

There is a reason why the U.S. Air Force consumes more than half of all the fuel used by the U.S. military: On an aircraft, every pound and inch count, and the plane has to be able to travel long distances with limited options for refueling. If you require extra weight for a less energy-dense fuel, that also leaves less weight for weapons and ammunition on a plane. A similar calculation holds for ships and the immense power required for their propulsion.

Second, while past downsizing of the U.S. military’s inventories of tanks, planes, and other weapons has reduced overall fuel demand, future energy demand will likely increase due to the higher demand of current and future weaponry. For instance, the F-35 and the littoral combat ship both use substantially more energy than the fighter plane and warship, respectively, that they were designed to replace. Greater use of electronics and communications on the battlefield also entails increased energy demand. In addition, energy is also now used as a weapon on the modern battlefield, such as electromagnetic pulse rifles used to defend against drones. Its use is likely to grow in the weapon systems that are now under development, including laser technology and other directed energy weapons.

Moreover, the U.S. military’s energy demand is likely to increase as it faces threats in multiple theaters and forward-deploys its forces more frequently. Only at noncombat installations has the Defense Department succeeded in keeping energy demand flat.

Third, militaries cannot easily transition to new fuel sources, since weapon and platform designs lock the military into specific fuel types for decades. For instance, a fighter plane designed to run on a certain grade of jet fuel derived from petroleum cannot be adapted easily to run on a different fuel.

A fourth reason that they will not transition away from fossil fuels any time soon is that militaries require a much higher degree of energy supply security than the civilian sector. In most conditions, a Tesla driver can tolerate being stranded with an empty battery on a highway, and entire cities have to deal with periodic blackouts. With obvious exceptions—such as hospitals and other vital infrastructure—civilian energy systems are designed to accept periodic energy disruptions in order to save costs.

In contrast, the military’s peak energy demand occurs when it is engaging in battle. Lack of reliable energy with ample backup—over long distances, in adverse weather conditions, and possibly in multiple locations simultaneously—translates to greater loss of life and possibly losing the battle or entire war. In future conflicts, adversaries will actively try to disrupt energy supplies, and thus the energy supply needs to be as reliable as possible. Liquid fuels derived from petroleum and nuclear-powered platforms provide the greatest reliability in this regard.

Militaries are not likely to use today’s dominant sources of renewable energy to power their operations. Renewable energy is generally used to produce electricity or liquid hydrogen, neither of which can be deployed today on a mass scale in the battlespace. Batteries struggle to reliably power passenger cars—let alone heavier vehicles. Hydrogen has insufficient energy density, and it’s even more explosively flammable than petroleum-derived fuels. That danger makes it especially unsuitable for fueling planes and ships.

Finally, today’s battlespace cannot be significantly electrified. Reliance on electricity increases vulnerability to cyberattacks, creates dependence on a foreign grid in the battlefield (versus depending on fuel supplies shipped in from elsewhere), and makes a force vulnerable to attacks on a long electricity supply line. Liquid fuel can be easily stored, while electricity has very limited options for storage. And long battery charging times are another barrier to use in the battlespace.

Access to energy supplies has been a decisive element in the outcome of battles and wars throughout history. In both world wars, the United States and its allies enjoyed a strategic advantage because the United States could meet its energy needs—and even supply its allies—from domestic energy sources. Given today’s wind and solar supply chains, a switch to higher consumption of renewable energy (even if it were possible) would foster dependence on materials and components imported from China or countries that China influences. This would erase one of the United States’ most fundamental strategic advantages: access to ample supplies of energy.

All that said, the U.S. military can indeed play a valuable role in an energy transition: as a source of innovation in the development and implementation of new energy technologies. Throughout history, militaries have played a major role in triggering energy transitions. The decision made by Winston Churchill, when he served as Britain’s first lord of the admiralty, to transition the Royal Navy from coal-fired propulsion to oil played a major role in the broader transition from coal to petroleum-derived fuels on the eve of World War I. During World War II, the development of nuclear weapons paved the way for the development of civilian nuclear energy.

It would be far better to task the U.S. military with coming up with a great new energy source than to try to hook it up to solar and wind. The U.S. Defense Department has the scale, budget, and laboratories for it to be a source of energy innovation that will impact not only the military itself, but also civilian energy use. Promising fields under development in the military are uncrewed vehicles, which could lower energy demand because of their smaller size and weight, and energy beaming, which could overcome some of the obstacles for using more electricity to power vehicles and weapons. Both the U.S. Navy and Air Force are researching and testing non-petroleum derived fuels. Research into nuclear micro-reactors, another carbon-free power source, is also underway.

Properly resourced, the military can play a key role in innovating an energy source to trigger a major energy transition. What it cannot do is transition based on current technologies, no matter how ardently administration officials try to advance a renewable energy agenda for defense. Militaries around the world will remain a large source for fossil fuel demand for the foreseeable future, and this should be taken into account when assessing the prospects of an energy transition.

Brenda Shaffer is a faculty member at the U.S. Naval Postgraduate School, a senior fellow at the Atlantic Council’s Global Energy Center, a senior advisor for energy at the Foundation for Defense of Democracies, and the co-author, with Alan Howard and Daniel Nussbaum, of Operational Energy, to be published in September.