Plan A is simply a modified version of the fleet's SIOP in the event of a second war between the Imperium and the stellar nations of humanity. A war which all military planners knew was an impossible task in the conventional field due to the vast difference in numbers between the Imperial Navy and the combined forces of humanity. While such a conflict would be devastating for the material and manpower stocks of the empire both in the actual combat and the fallout and subsequent occupations the math simply stated the obvious, they would succeed even if the victory was pyrrhic in the long term.
For humanity as a whole the thought of their destiny and sovereignty becoming subjects of Imperial whim was simply an unacceptable option and mutual plans where drawn up. Like humanity of the twentieth century and the 23rd century the tendency to move toward the promise of mutually assured destruction was an old hand and one all decided to put on the table before the Imperium. The complete promise that should human independence fall that all ability and capability of its strategic systems would be turned on the Imperium itself.
Notable Systems: