War is a matter of both the head and the heart.
So there were pragmatic reasons for striking at them. For a start, Fronen was simply really, really big; double the size of Sint or Old Grehk and three times the size of Thalmarkin even before they took Dyomoque. One badly timed rebellion or a foolish ruler and the north could easily have belonged to Fronen if they were smart. They had five rich regions sitting on the border with Thalmarkin and OG, meaning there was no need to TO rurals (which we've already got in spades). Fronen's only close ally was/is Enweil and keeping them out of the fight was/is nice and easy.
But there were plenty of less calculating things behind the decision. As Lorgan's already said, the north was more of less abandoned by Fronen during the Invasion. OG had 5 nobles die at the hands of the Undead, about a fifth of our characters. At one stage we lost our only city and capital, Ossmat, and were down to a grand total of four regions (Vatrona, Mekoter, Junohep and Xerus). We only survived because we surrendered to the Undead. Even after we surrendered, OG quietly supplied Thalmarkin and Sint with food. Jack Carnes brought 50 archers to help defend Ossmat, that was the only help we got from Fronen. Meanwhile, the two regions Fronen lost during the whole invasion were Yipinalke and Jedinchel; neither was lost for more than a week or two and OG actually helped them reclaim both.
You could say we're a touch bitter. And thats not even getting into the Fronen-OG wars that came before that...
