ignominia ad Orientem, legionibus in Armenia sub iugum missis aegreque Syria retenta.
The primary issue with this section is the phrase in the middle, legionibus in Armenia sub iugum missis. To begin with, this is an ablative absolute, describing a military disaster.
A frequent translation of this seems to be that the legions have been sent into Armena in order to subjugate the people there, taking the words effectively as legionibus missis in Armenia sub iugum; this reading does not work. First, in this case, in would have to take the accusative rather than the ablative as it actually appears. Further, mitto sub iugum is an idiom, referencing “an arrangement of two vertical and one transverse spear under which a conquered army was made to pass” [OLD iugum 5]. Taking the idiom collectively to mean “conquered,” “defeated” or “subjugated” conveys the sense of this quite effectively.
Thus in order to accurately deal with the Latin presented here, the order legionibus in Armenia missis sub iugum is better; that is, the legions in Armenia, whether recently sent there or stationed there on a permanent basis, have been subjugated. Alternately, in Armenia could simply be describing where they suffered their defeat, though that would be ignoring the positioning of the prepositional phrase between the noun and the participle modifying it, which seems ill-advised when it reads well as written.
Once the meaning of the ablative absolute has been understood, the preceding and following comments are given important context: the loss of the legions is the shame in the East, and Syria is barely held onto because the troops tasked with that have been killed or enslaved.