The uneasy peace
When the Chestnut war started, President Laforge ruled over Ostark. Laforge was, by most opinions, an inadequate leader who used his position mostly to profit himself. Despite this, Ostark prospered under his 'reign', perhaps because many of his colleagues possessed the qualities he lacked. In any event, having Laforge as the president didn't weaken the war effort and Ostark dominated Nirnivia on the front.
It is not Laforge who is the important person here, but rather the one he made his vice-president: A mysterious figure that forsook his name. The Ostarkirans called him 'The Good Doctor' due to the fact he excelled at medicine, while once the Nirnivians learned of him, they chose the moniker 'Doctor Death'. Physically striking would be an understatement as a description for the vice-president. He lost most of his body, his limbs, and torso, replaced by cybernetic parts. Only his head remained of his original body, and even then, a speaker replaced his mouth and an electronic eye replaced his right one. Yet, the truly mesmerizing quality of the doctor wasn't his appearance, but rather his mind. He possessed an incredible intelligence and managed to read people like books, often predicting their behavior with a ludicrous precision. This proved true of even Daniel Ricdeau, the current Commander of N.I.S.D.A., and as such, the Doctor's advice on military matters granted Ostark even more victories than they would normally have. It is assumed that even without the vice-president's support, Ostark would have won the conflict, but with his aid, they dominated. The doctor provided counsel on both military and domestic matters and Laforge soon learned he should listen to him no matter the subject.
One day, Laforge contracted an incurable disease and eventually died. Legally, the doctor was his successor and he thus became president. Almost immediately, the doctor scaled down the war effort. He set up a truce with Nirnivia so they could discuss peace terms. During their meetings, he explained Ostark had some demands and should they be met he would withdraw Ostarkiran troops from Nirnivia. Most of his demands were deemed reasonable. He wanted the two countries to have a way to communicate in order to avoid future conflicts such as the Chestnut war. They would remain separated and isolated, but now with a direct line of communication between his presidency, the Nirnivian council and N.I.S.D.A. Nirnivia agreed as the suggestion seemed valid given recent events. Maybe pure isolation wasn't adequate after all. His other demands were for the most part trivial: he wanted access to a small part of the neutral zone sitting at the Ostarkiran border and other minor details. The Nirnivians accepted these too. His last demand, however, ended up being ridiculous; something Nirnivia could never consent to. They refused and further negotiations failed to change either side's mind. Fear of a renewed conflict arose, but despite the Nirnivians' refusal, the doctor withdrew his troops, claiming the war caused enough damage and he wished to resolve this matter through diplomacy, even if it took years. As long as Nirnivians didn't attack them in retaliation, they'd be left alone until the matter resolved, or at least until he changed his mind. This period became known as the uneasy peace. Officially, no battles were waged, but N.I.S.D.A lived in constant fear of another invasion. Thanks to the new means of communication, negotiations continued, but neither side would budge and it soon became clear it was useless and the discussions pretty much stopped. Yet, the doctor didn't attack. Well, actually he did, some small-scale assaults were mounted in Nirnivia in an attempt to fulfill his last demand by force, but these were minor and there wasn't any proof they were committed by Ostarkirans, though who else could be behind them? Nirnivia suffered these attacks without retaliation. Each time they happened, they started a discussion where the doctor denied any involvement and Nirnivia begrudgingly pretended to accept this as true, given another war would surely bring their destruction. Still, everyone at N.I.S.D.A knew another conflict was inevitable, they simply bided their time in the hope that an opportunity to come out on top would appear in the future.
BBR
Not every Nirnivian agreed with the uneasy peace. Some felt they played into Ostark and the doctor's hands. Since the uneasy peace was an idea from the doctor, no doubt it benefited him in some way, so his wish should not be granted. Eventually, some of these people formed a terrorist group named Broken Beyond Repair, or BBR for short. They mostly built their outposts in the neutral zone, near Ostark, so they couldn't be easily stopped thanks to the isolation rule. They still had a presence within both countries though. They often employed guerilla tactics against Ostark in an effort to weaken them, but for the most part, they were merely a nuisance and not a real threat. They hoped to change this fact, however. Many rich Nirnivians shared BBR's view, though not publicly, and secretly funded the group explaining how it managed to arm and supply itself.
Officially, N.I.S.D.A and the council denounced BBR as terrorists and accused them of risking breaking the peace, though in private, many N.I.S.D.A officers sympathized with them, at least to a point. To prevent war, N.I.S.D.A agreed to help Ostark stop BBR. The isolation rule had been changed so raids against BBR outposts were permitted as long as proper permission from the other side was granted, and both Ostark and Nirnivia often assaulted their cells. Despite their joined effort, however, BBR survived and kept stinging Ostark just enough to be a thorn in their side.