Most countries do not have enough to independently supply their own energy needs from fossil fuels, and they need to import more or less continuously, while renewable energy infrastructure needs building out and then lasts a very long time. This massively affects the strategic effects of losing your supply: one becomes an acute problem very quickly while the other is still a problem but it takes a lot longer to become a real problem.

I don't think the main focus is on converting military operations themselves to work without fossil fuels, but more that if you can convert your civilian industry then you have a much better overall strategic position because your total demand for fossil fuels is much lower. (Strategy is a lot more than just your military)