There are some conflicting information about Arden.
According to the maps from the essay about the settlement of the Marches, Arden was settled before 300 but remains independent until after 500. [There are three maps showing settlements in 300, 400, and 500. Arden is settled but outside the Imperial border on all three.]
According to the description on p. 8 of SMC, Arden was part of the Imperium from 290 to 986. It became independent as part of the armistice following 3FW. In 1110 it has "recently" become imperialistic and has taken over Zircon and Utoland. "Recently" is before 1105, though, as Utoland belongs to Arden already.
According to the historical maps in SMC, Arden became part of the Imperium during 1FW and becomes independent during 3FW.
Before 5FW, Arden is ostensibly a democracy but actually a charismatic oligarchy; government officials are supposedly elected democratically, but in practice, candidates can only be members of the Arden Society, a small group of dedicated individuals who become members through financial or labor contributions. [SOA:2]
In 1110 the local pro-Zhodani government (i.e. the Arden Society) was replaced by a pro-Imperial coalition representing commercial interests both on and off-world. [TNS]
Some time between 1110 and 1120, the aforementioned government is replaced by a different charismatic oligarchy: a hereditary noble class. [BtC].
So far canon. I've been working on Arden's history a bit, although I'm far from finished. Here's what I have so far (None of it canonical):
Arden was first settled around -100 by a group of refugees from Hofud fleeing the chaos of the War of the First Rebellion. The refugees survived and even multiplied, but they lost most of their technology and all of their written history. Their descendants, somewhat more than 100,000 people calling themselves the Hrolfungs, were found in 56 by the Scout cruiser Vitus Bering. Following standard procedure the Scouts put the world under administrative interdict pending further investigation, but due to limited resources no follow-up expedition was arranged for 40 years.
In 96, an Imperial transport fleet carrying some 24,000 exiles from Warwick (Massilia 2109, later named Murcia) ignored the interdict and deposited its passengers in an empty part of the world a few hundred miles from the populated region. At this time the Hrolfungs is estimated to have numbered in excess of 350,000.
The new colonists called the world New Warwick and the lush, forested location of their first settlement Arden. The original name quickly fell into disuse and the world eventually became know as Arden. The leaders of the colony were neo-romanticists and tended to name geographical features of their new home from Shakespeare's plays, mostly the comedies.
ca. -100 Sword Worlder refugees settle on Arden and quickly regress
to TTL 2.
56 Arden found by Scout cruiser VITUS BERING and put under
administrative interdict.
96 Settlement of Arden by Pacification Campaign exiles from
Murcia (Massilia 2109).
290 Arden becomes Imperial client state.
470 [A big chunk of Vilis joins the Imperium en bloc, but Arden
doesn't join until 1FW].
511-530 RIO Movement colonizes Rio (Spinward Marches 0301).
Early 600s Arden becomes part of the Imperium. Local leader made Marquis
of Arden.
629 As part of Arbellatra's political reorganization, the Marquis of Arden is
elevated to Duke of Arden and made subsector duke of Vilis. The Count
of Vilis is moved to Antares and replaced by a noble from Massila.
986 Arden becomes independent as part of the armistice following
3FW. Followed shortly after by balkanization.
986 The Duke of Arden receives land on Regina and Efate as compensation for
the loss of his personal estates and is "invited" to move to Capital.
1029 Arden Society founded with the goal of reunifying Arden.
1072-1083 The different national chapters of the Arden Society force
local governments to join in a Federation.
1105 Arden is straining under internal pressures. Government turns
imperialist to divert attention.
1106 Takeover (not colonization) of Utoland.
Early 1107 Takeover (not colonization) of Zircon.
1110 Government toppled, Imperium-friendly (Megacorporation-
influenced) government installed.
1112 Merchant government in turn toppled by aristocrats.
1113 Federation resumes expansion. Takeover of Zenopit.
That's as far as I've come, except for some scattered historical notes. Make whatever use of it you like.
Hans