As has been mentioned, the old Imperium managed to keep intact and relatively unravaged the core worlds of the Empire. Yet, for over a complete Cycle, it was all that they could manage merely to avoid being sucked up in the petty squabbles of the post-Imperial "Successor States". Most of the core worlds were almost totally depleted of any significant raw materials, and this also had a significant effect on the posture of the rump Empire in the immediate post collapse period. The main problem was that the Old Imperium (as the Forerunners came to call it latter, we generally refer to it as the "First Imperium" or the "Old Imperium") had reached a level of technology where the artefacts - especially personal weapons and FTL ships were practically indestructible! Thus, though almost all worlds except the core around Home had actually lost the capability to BUILD such ships (or even to repair them), and were little better than prescientific Barbarians in most cases, they continued to be able to carry on raiding, conquering, looting and pillaging the starlanes well beyond the nominal date for establishment of the next Forerunner Interstellar State sometime in the 6th Cbluvoe of the eleventh Cycle. In fact, amazing as it might seem to us, there are Forerunner records that show that instances of still operable "Old Imperium" technology were still being commonly encountered in isolated areas of the old Imperial sphere well into the TWENTIETH Cycle! This being some ONE HUNDRED THOUSAND YEARS after they were first constructed!
For this an other reasons, the Rump Imperium led a very precarious existence - until the "Phoenix Rebellion" of 3/17/12/19/20/20/20/20/20 in the fourteenth Cycle. Again, we translate the single Forerunner Glyph of a winged and beaked reptile emerging from a fiercely burning pyre as being the equivalent of the Terran legend of the "Phoenix". In fact, some distinguished archaeologists and anthropologists insist that the Terran legend - and similar legends on countless other planets - is a race memory of the Forerunner "Phoenix Age" and the so-called "Phoenix Confederation" - though this is, of course, subject to considerable dispute.
However, with the elimination of the decadent Imperial administration by the Phoenix Rebels (legend has it that the last Emperor of the Old Imperium was the ringleader of the Rebellion, but this may be an apocryphal story), the rump Forerunner society was rapidly revitalised, the dead weight of the old Imperial Bureaucracy disbanded and replaced by more streamlined and efficient systems. Technology was also revitalised - the rump Imperium had never lost FTL capability, though towards the end of its life, immediately before the revolution, there are some signs that it had lost the ability to do anything more than repair the still existing FTL ships of the Old Imperium from ever dwindling existing stocks of spares. Whatever the case, the early years of the Phoenix Confederation were ones of rapid regaining of the "lost" - perhaps "mislaid" would be a more appropriate term - technologies of the "Golden Age" of the Old Imperium
During the early years of the rebirth of sciences (as opposed to technology), the Phoenix Confederation strove to adopt a Constitution that would be capable of withstanding the massive problem of administering a large Interstellar state. The one finally adopted was a compilation - so our sources state - of the best features of the Consilium period and the Old Imperium. From various clues we have, we have been able to deduce that the most probable form taken by this reorganised State was that we today would call a Confederation. That is, a Central Government that is generally responsible for the relationship between member states, for defence of Confederation Space, and for generally seeing that everyone within the Confederate sphere played by the "rules of the game". The Confederacy was thus much more easy going than either the Consilium or the Imperium had been.
The Phoenix Confederation seemed to take the view that whatever form of government a member state chose - and by whatever means it was chosen - it was none of the Confederacy's business - as long as the relatively minimal Confederal taxes were paid and the required minimum Starforce units for planetary defence provided. If a given planet was ruled by a Tyrannical Dictator who ruled by virtue of a ruthlessly efficient and vicious Secret police, it was all the same to the Confederate Government as a planet ruled by an Athenian Democracy. The Confederacy was even obliged by its constitution to defend local RECOGNISED Governments from rebels - and took this requirement VERY seriously; of course, there was always the problem of gathering enough forces to do the job. Strangely enough, if a Democratic government was threatened by totalitarians rebels, response time was lightning swift; yet, when a Totalitarian regime was threatened by Democratic rebels, the Confederate Starforces seemed only to arrive in time to recognise the NEW Government!
Of course, this meant that many less than popular Planetary Governments were very reluctant to join. Especially as the Confederacy disavowed all foreign conquest - only accepting voluntary members. The hedge here was that, while CONFEDERATE Starforce would not attempt such illegal acts, they also would not protect non-members from any "unpleasantness" between a member state (or states) of the Confederacy - that would be interfering with the internal affairs of a Member Planet! Another hedge was that Confederate Mercantile law ONLY APPLIED to Confederate citizens - in all other cases, such as contracts between Confederate members and independent worlds, the Contract was subject to the law of the planet on which it was made - or, more often, on which it was attempted to enforce it. This was obviously open to abuse by Confederate Members to the disadvantage of non-members but, as the Confederacy always pointed out - Confederate membership in such case solved any problems there may have been (sometimes even retroactively, by local negotiation).
Despite all this machiavellian diplomatic manoeuvring, there was much more behind the expansion of the Phoenix Confederacy. Part of it was due to the fact that the Confederate Homeworlds (the rump of the Old Imperium - as well as Home itself) had, unlike the vast majority of the other ex-Imperial worlds, managed to retain most of the old pre-interregnum technology intact, and had quickly relearned what had been lost. This made trade with the Confederacy quite attractive to the successor states - and it had to be carried out on the Confederacy's terms since there were so few non-Confederate systems that had retained Spaceflight, let alone the advanced technologies necessary to build and maintain FTL ships. The relatively few remaining ships of the Old Imperium were often encountered as "Tramp" freight carriers - but were too widely scattered to be a tool to use against the growing economic clout of the Confederacy. It was, in fact, this lure of trade coupled with the "Pax Phoenixus" (i.e. the assurance of protection for EVERY member of the Confederacy from outside attack) that made Confederate membership so attractive to many worlds.
Even so, it was took almost a two full Cycles for the Confederacy to expand to fill the sphere covered by even the old Consilium. The keyword, of course, was CO-OPERATION - and it is this that made the Confederacy so much more stable in the long term than either the Consilium or the Old Empire. It is also why it took so long for it to reach even this level of its predecessor's former glory. The expansionistic period was, however, much more due to the desire of individual planets to join than simply out and put conquest. This, coupled with the benefits of membership and the hazards of non-membership, actually attracted voluntary application for membership.
So, by the end of the sixteenth Cycle, the Forerunners were still in an expansionistic stage of building a star-spanning culture. This continues into the seventeenth Cycle, and beyond.