If you don't want to have a global AI on the human side, consider the possibility of an alien race that created such an AI and didn't make it friendly and was wiped out or enslaved. Perhaps they programmed it to take over the universe for them and it took over them too. In any case it is programmed to take over the universe and is willing to do whatever it needs to in order to accomplish this task. Presumably the AI would either have to posess a form of instantaneous communication, in which case it would have access to all of the processing power of every processor connected to it at all times, and would have instant knowledge of everything that was known by system connected to it, making it effectively omnicient within its own domain. Otherwise it would need to be able to have portions of its conciousness break away and operate independently and then rejoin the whole. In either case loss of any portion of it is relevant only in that the resources that went into producing it are wasted.

 

Because it needs no organic components, it is limited in its military power only by the material and energy resources it has access to and the time needed to process those resources into a final form. It may well have access to nanotechnology which allows it to process materals at the molecular and atomic scale, and in fact all materials it makes may be built this way. Note that its material resources include all of the planets in its domain no matter how uninhabitable and its energy resources may include a significant fraction of the energy produced by the stars in its domain. Science fiction writers have speculated that a star could be surrounded by solar collectors. Note that such a large solar collector array would also act as a solar sail, so it would have to be extremely strong, and would have to symetrically surround the star. Energy could be transmitted by using it to generate a laser beam and the beam could then be directed to wherever in a system that energy is needed. It would also be a massively powerful weapon.