The military hurts when fuel costs rise. Why haven't they yet R&D'ed hybrid drivetrains onto their vehicles?

With combat equipment fuel economy isn’t a consideration. These vehicles are heavy some of them measured in tens of tons in weight. Power ,speed, and reliability come first. Some of the non tactical fleet may have been green for some time. Have you checked into that? For the same reason the fire departments need power and reliability because they are in the business of being in emergencies.

Reliability and toughness.

Most hybrids require Lithium batteries, which are a HUGE liability in a combat vehicle. You don’t see firefighters strapping on sticks of dynamite and dousing themselves in gasoline before entering a burning building, do you? You have to remember that military vehicles face conditions most Priuses never will. When a hybrid/EV gets into an accident, there is a chance of a battery fire, and those are harder to deal with than a gasoline fire (though many military vehicles are diesel, partly for that reason). A normal car is less likely to get into an incident than an Humvee driving through Iraq; the average commuter rarely encounters gunfire or IEDs. The risk is acceptable in a civilian vehicle, but not on the frontline.

There is also the fact that hybrid technology isn’t exactly proven. Many hybrid vehicles have had various issues as they work out the bugs of this emerging technology, and that is absolutely unacceptable for a combat vehicle.

Not to mention they are more complex and thus maintenance/repair becomes more difficult. With an old-tech engine, you can usually kludge a field-expedient repair together using a Leatherman and random junk in the trunk whereas hybrids need a garage (at least) for even the simplest things. Again, utterly unacceptable for a combat vehicle.

The US Army has had hybrid-diesel Humvees since at least 2006 that I know of. The ones I saw then were prototypes, and the presenter at that time stated that those vehicles would be in production soon. I don’t know if this ever actually took place.

@jerv Humvees are already far too complex for ordinary field repairs of the type you describe. Also, Civilian Hummers have an ordinary fuel tank (as opposed to a fire-resistant fuel ‘cell’ like racing vehicles have). This is potentially a bomb dwarfing any conceivable hybrid battery pack in destructive energy and power.

The biggest logistical problem with hybrids, as I see it, is the added weight of the additional drive components. Batteries are getting lighter all the time, but a pack big enough to be useful in a hybrid is still quite heavy- and the heavier the vehicle, the heavier the batteries need to be. That weight could be more effectively used for armor or cargo capacity in combat conditions.

Because the military is the right wing’s pet and hybrids are for communists.

Hybrids are an engineering marvel but Waaaay too complicated for practical military uses. Simple, easy to maintain and repair are a tactical advantage. A hybrid is a liability. Most heavy military equipment are diesel and can drink about anything available.