I think your last paragraph says it all.
' And that was constantly under the THREAT of invasion etc...'
I dont recall thakria being instantly reinvaded when it fell, I believe that Ragar is trying to avoid that happening to Mercinae when the treaty ends.
But then again my memory is a little fuzzy, I seem to remember that as soon as Springdale (and i shall call it that as it was its name at the time) was
a city large enough to compete with the others in open warfare Parrius made a flip to the side of Thakria...
Which means that Thakria wouldnt have been under threat from three cities...
I could also point out that Mercinae is situated right in the middle of Parrius and Thakria making it quite an easy target where as in the past for Parrius to attack Thakria or vice versa
(when such i think would have happened) a far greater effort was required, but i won't point that out cos you are probably bored of this post already by now.
Written by my hand on the 13th of Midwinter, in the year 1034.