Actually its roughly true....if you count all militias: 10k against 100k.
Still, think of it this way: Guillaume managed to aggravate, betray, shortchange and outright lie to enough people that ALL those came to kill him. I (as player) think that if Guillaume would not have rejoined Enweil then it would have had a good chance of survival. With him as de-facto Enweils leader...very little. My char Sassan is having a rage-fit each time Guillaumes name is mentioned...
Note that I as player heartily applaud Guillaumes achievement. There are far to many more-or-less honorable do-gooders (like Sassan) and too few villains.
I had honestly forgotten about the betrayal of Melhed, I was clueless as to why the heck they were there. That's how insignificant that treason had been, from my perspective. With hindsight, I should have either not done any harm to Melhed, or done a lot more. The Prince had it right. A small betrayal will piss off as much as a big one, so if you are to betray, you make as well make it bad enough that they can't get revenge. Draining all of Melhed's food might have given Fronen enough of an edge to win. My bad. :/
As for Riombara, they declared war on Enweil when Guillaume was a noble of Melhed. There's really no reason to believe that Riombara would ever stop attacking Enweil. Guillaume might have brought one more enemy to Enweil, but he also managed to make it survive that long despite the odds. Riombara and Enweil had a history that predates Guillaume significantly, and for as long as I've been playing on BT, Rio was always the one to declare war. Hard to take much credit, here.
And Ar Agyr... had little to do with Guillaume. That's just the leader with a 10-year-old vendetta, allying with a realm that did what the realm he is destroying over.
So really, I only take credit for Melhed's involvement. As for the rest of Enweil's defeat, I place that largely on the invasions, which bled Enweil considerably each time, while leaving Riombara stronger each time. Good diplomacy or bad diplomacy would have changed little to this, and there was no way to factor how invasions would turn out and how far-apart they would be while doing the scheming. Might makes right. Had the invasions turned out different, Guillaume would never had had to exile himself (and thus betray Melhed), and he'd be seen as a hero by many, while the history books would have put down Riombara as the villain.
Sure, Guillaume did a ton of mistakes along the way, and ultimately failed in his ambitions, but much of the current events are beyond his doing.