Energy considerations are core to mission delivery of armed forces worldwide. The interaction between military energy issues and non-military energy issues is not often explicitly treated in the literature or media, although in the last decade there has been some increase driven especially by the issues of clean energy. It is recognized that the military has for more than a hundred years taken a leadership role in terms on research and development (R&D) of specific energy technologies - most commonly where they are applicable in theater. More recently that R&D leadership has moved to the energy efficiency of home-country bases, and the development of renewable energy projects for areas as diverse as mini-grids for in-country installations, to alternative fuels for submarines and jets. Nevertheless, the military in most major countries tends to see energy issues as a matter of mission delivery or conversely the denial of enemy energy supply chains as a source of advantage. In this paper we explore the evolving relationship between energy issues and defense planning, and show how these developments have implications for military tactics and strategy and for civil energy policy.