Third Founding
The Third Founding of Space Marine Chapters was initiated by the Primarch Rogal Dorn in 001.M32.
It was particularly notable because it was genuinely the first time that the Space Marine ranks were given explicit permission to expand since the Great Crusade; the Second Founding was merely a reorganisation of the remaining manpower and materiel from the Legions that remained after the Horus Heresy and only loosely constituted a "founding" when compared to the tradition of new Chapters being drafted in later centuries. The Third Founding would have necessitated not only the addition of new Space Marines, but of new ships and wargear to equip them with.
Although though the 21st Founding is considered the largest founding ever, the Third Founding is especially notable because the number of Space Marines Chapters virtually doubled in an extremely short space of time. Over one hundred writs of succession were issued as part of the Founding, which when one considers that by the end of the 41st Millennium there were roughly a thousand chapters it constitutes a tremendous jump in the total Space Marine population. (Also remember that many of the 21st Founding Chapters were wiped out due to unacceptable Gene-Seed flaws, which would counter any long term rise in the Space Marine population.)
It also marked the first time that these new "Successor" chapters would be given successors of their own, and thereby expanding the lines of descent for both Gene-Seed bloodlines, so some of these new Chapters would have only a distant connection to their First Founding Legion progenitor, skewed by the varied warrior traditions of their Second Founding fathers. Only nine writs of succession (ie: 9%) were granted to the Ultramarines directly, but considering that the Ultramarines had the greatest number of Second Founding successors one can assume the Sons of Guilliman were proportionally represented.
It also caught some by surprise, hinting towards a mistakenly held belief that Guilliman's Second Founding was the be-all-end-all for the Space Marines. Roboute Guilliman had been indisposed for nearly nine-hundred years and his position of Lord Commander of the Imperium had passed into mortal hands. Even though the position was Rogal Dorn's during the Horus Heresy, his brother apparently did not take up the position since the post was still referred to as "Lord Guilliman" well into M32, quite possibly due to a lingering anti-Astartes resentment left over from the Siege of Terra itself; Dorn had to petition the High Lords of Terra in order to get the motion passed, rather than make a proclamation or decree.
Because the Founding was announced unexpectedly, it caused the Ultramarines to scramble to conceal the evidence of their Primarch's breach of the Codex Astartes when he left an extra company in place to cover up the completely separate issue of the Imperium Secundus. The company in question was rolled into its own Chapter, forever disconnecting itself from the identity and heritage of its parent legion.
The precedent set by the Third Founding was followed over fife hundred years later by the Fourth Founding; initiated by Lord Commander Maximus Thane shortly after the War of the Beast in 546.M32.
Third Founding Chapters