Civil War (S8 LD A-M, p. 20-21)
Includes that the right of Assassination was precedential in YI 244, and repeatedly during the Emperors of the Flag sub-part of the Civil War
The Civil War dates from 604 to 622 ... which for all intents and purposes means "500 years ago" relative to the reign of Strephon. In other words, the precedent was there (granted), but hadn't been used (successfully) for centuries. The library data entry notes that of the 18 emperors (of the flag) during the Civil War period, 7 were assassinated, 10 were killed in battle, and only one survived (the last one standing). Arbellatra broke with precedent after her fleet drove to Capital by not declaring herself Emperor, but instead declaring herself Regent while a suitable heir could be found. Eventually, the imperial bureaucracy approached Arbellatra to take the Iridium Throne herself (after 7 years of regency). Following the Civil War, the Express Boat system was organized and put into operation, limiting the potential for fragmentation due to the sheer distance of far flung borders in the future.
Emperors of the Flag (S8 LD A-M, p. 26) reiterates the right of Assassination. It also reiterates the regency of Arbellatra being the end of the civil war
Basically summarizing the Civil War entry for our purposes here. Notes that in 619 there were 6 emperors, none of which served longer than 3 months, so there was a LOT of turmoil and turnover at the top.
Emperors List (S8 LD A-M, p. 44-46)
Gives a bunch of interesting bits... but stops short of saying Porfiria having been the Assassin of Cleon III, aka Cleon the Mad.
Cleon III
Born in 201, proclaimed emperor in 244, assassinated in 245. A decision to remove him began shortly after his ascension to the Iridium Throne. The use of assassination to "dispose" of an emperor in this case was intended to be a legitimation of a one-time event, that then got revived during the Civil War.
The weakness politically of the Archdukes was presented in the essay Imperial Space (S11 LD N-Z, p 6-7)
The archdukes can create Knights and Baronets (subject only to disapproval by the Emperor). They maintain their hereditary lands and incomes, which would presumably be CONSIDERABLE(!)
o:. They also serve as surrogates for the Emperor at functions the Emperor cannot attend in person due to distance.
The existence of the 5 Archdukes are mentioned in the essay, Imperial Ranks (S 11 LD N-Z, p 34-36), the relevant section being on p36
An archdukedom includes a fief consisting of an entire world, generally retained as a private reserve. Fiefs are granted to individuals at the discretion of the emperor and may be inherited by the archduke's heirs, but remain the property of the Emperor. The fief grant conveys the right to use the land, rent or lease it out or otherwise collect money from it. The fief is a convenient way for the Emperor to reward certain nobles.
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In terms of saying "assassinations have happened before in imperial history" ... I suppose these library entries support the notion.
In terms of saying "therefore it was reasonable to assume it would happen to Strephon NOW" is a stretch of interplanetary (if not interstellar) proportions. I mean, the whole emperors of the flag and Civil War era was 500 years ago.
Assassinations are usually (not always, but usually) the result of some kind of political turmoil or tumultuous times. Things are BAD and then you get the assassination(s). It's not typical for things to be doing okay and then there's an assassination (out of nowhere) that then kicks off an era of political turmoil and tumultuous times.
I mean, where were all the hints that there was something rotten in Denmar- ... I mean at Core ... before the shock surprise of the assassination happened? This is why people (playing the game) felt kind of blindsided by this turn of events in the OTU, and why it seems like it was something imposed onto the game's timeline from outside the game universe itself, rather than being something that made a lot of sense from the "inside" as a kind of natural development with the way things were heading. The fact that these events "happened" in the OTU just in time for a new edition of Traveller to be released only helps cement that impression as being the real motivation behind the turn of events.
Cart vs Horse
Round 1
FIGHT!
In other words, if the setup had been better then the acceptance of the turn of events in the storytelling would be higher.