EDIT: It's also good to note that Fontan's fall really began when Krimml seceeded. That was a funny episode.
Fontan's fall really began when the MoD decided it was the government and was at liberty to create new realms without discussing the matter with the whole realm. The Civil War, the birth of Westmoor and the inability of successive governments to make peace with Sirion all hung on that single decision.
That's not to say something else wouldn't have lead to a similar outcome
