Most defensible, not most powerful. Significant difference. Especially with the new estate/tax system. Didn't a bunch of Sirion's regions suddenly become much less profitable?
In truth, all it'd take is a duke (or the lords of several key regions) seceding or joining a neighboring realm and suddenly Sirion has a huge gap in its armor.
It's funny though, for as much as Sirion tries to portray others as evil/dishonorable, they seem to forget that the ganged-up war against them was due to them trying to sell out other realms (including Westmoor) out to Fontan.
...but I'm sure there was no legitimate reason for realms declaring war on Sirion when it was betraying its allies. 
Westmoor an ally? Please.
And the reason all those realms went to war with Sirion is because they wanted to fight in a war. Not because of some(most likely fabricated) excuse that Sirion was selling them out.
Sirion is neck and neck with Perdan population and food wise, produces the most gold, has the most nobles, is the most defensible, and probably has the largest mobile army and your telling me they AREN'T the strongest?
And just about any realm would have major defensive problems if a whole duchy seceded from their nation. But I suppose all realms die eventually. Fontan was the top dog at one point too.
