OT Warfare after the last drop of oil.

[Urban]

Actually vegetal substitutes are the better solution. They could be used for diesel engines with little modifications.

For the fuel cells even if it's technically possible, the fuel needed (methanol, hygrogen and other low molecular weight compound) are not truly useful for military. To highly volatile,flammable and/or explosive.
Special dedicace for the hydrogen, this nasty thing could easily pass through metallics and plastic tanks .