Their fatigues aren't the only thing green about them now.

Throughout history, armies have historically been a hugely significant expenditure of resources. Whether it be materials to produce weapons, or the food required to feed a large host of soldiers, a military is always going to be difficult to maintain. In our modern era, one of the prime examples of this is the military demand for fuel, to operate most of their machinery. Whether it simply be operating equipment around the base, or keeping a line of tanks moving toward an objective, fuel is consumed in vast quantities by the branches of the military. And while they only account for a small percentage of worldwide demand, the US military is still one of the largest consumers of petroleum in the world, consuming about 340,000 barrels per day. While there seem to be those who would claim that this isn't enough consumption to change the alternative energy market, they are basing these arguments on current status of alternative fuel production, and the fact that there isn't much research into making it cheaper or better. If a large consumer like the military were to begin putting money toward alternative energy sources, even if it is by buying it instead of funding research, I'm sure that some sort of headway would be made by the extra cash flow that companies would receive if they had a reliable customer. One could argue that they played a role in popularizing the markets for coal, gas, and nuclear power as these came into play. Besides, it can be easier to enact change through purchasing power when a select few are the ones receiving the money to produce something, and there aren't that many biofuel companies currently operating at commercial capacity.
"'You need that big anchor customer. And the Navy can afford a premium, because it knows how much petroleum really costs,' explains Brook Porter, an investment partner at the venture capital firm Kleiner Perkins Caufield & Byers, which has put more than $1.5 billion into so-called 'clean tech' companies. For some of these firms, a big military contract could mean the difference between life and death."

While I do think that energy sources other than combustible fuels would be an optimal choice environmentally, it is hard to deny the power and usefulness of liquid combustibles. Many military vehicles get less than a mile to a gallon of fuel, but while they do need to try and improve efficiency to improve operating times and the weight of fuel needed to travel, this means they probably can't switch to most other types of alternative energy, as they wouldn't be able to provide the power needed to, say, fly a multi-ton aircraft carrying a multi-ton tank  over 26,000 miles. However, there are other applications for other alternative energy sources, which the marines have been pushing research for research and development into self-sufficient power sources to keep equipment running, which besides reducing reliance on batteries from a supply train, the weight carried by a "patrol of 35 soldiers shed[s] 700 pounds." So really, this brings back a talking point that I often think needs to be addressed whenever alternative energy is discussed: There is no one solution for everything. One of the beautiful things about the alternative energy sources being developed is that there is a variety. Something that doesn't work in one place can be switched out with another type, or weaknesses in one system can be compensated with an additional system being available. For example, solar cells might not work as well in a rainforest environment, with the thick foliage and lack of constant sun. But if there were a system that could take advantage of all the water that is readily available in such an environment, such as a hydrogen fuel cell generator that could produce the hydrogen with electrolysis from a hand crank generator (may not help with battery-type needs, but just throwing something out there). So, while the military should look into improving fuel efficiency and alternative fuels for their vehicle fleet, they should look into every field to try and find ways to reduce the need for resupplies and reliance on some other producer.

 Something that the military does understand is that they do have a problem with fossil fuels, though not on an environmental aspect. Relying on a foreign source for such an essential resource is a tactical vulnerability of the worst kind. But we are so deep in our reliance on fossil fuels that ways to combat said reliance are difficult, and viewed as impossible by many conservative individuals, though whether that is due to actual perceived difficulty or oil lobbyists is a different issue. In fact, there is a bill in the works to make it so that alternative fuels practically can't be bought or produced by the military, if they exceed the cost of an equivalent amount of fossil fuel. The fact that the military was testing large scale use of alternative fuels, instead of making a push to a full transition, may be an indication that this is a push by oil lobbyists. The Air Force was testing a much more expensive fuel than the "Great Green Fleet" that prompted the bill, but because they were doing it on a smaller scale it slipped under the radar. But those who participate in such testing are of the firm belief that the US needs to try and improve energy self-reliance, and are trying to take steps to make the transition easier and sooner than civilian economics can.

With the way things are going, we need to make a shift away from petroleum fuels as the primary energy source, even if we don't simply get rid of them. However, if we want to have them as an alternative in case something happens in the future, we need to start the shift now. The military taking an interest in this matter may seem like an odd place to start, given the destructive nature of such a body, but their purchasing power is unrivaled in terms of getting high tech and experimental resources. Besides, if they can manage to reduce reliance on petroleum, then it may be a pretty good indicator that anyone can.