The basic history of my two countries. This is the very earliest version, and there are some things that need working out, but I'm satisfied with the premise.

Morancis and The Great Forest used to be separate countries. Actually, The Great Forest was not a really a country, but a bunch of small, isolated autonomous villages. But then they all are threatened by an unknown evil. (Unknown even to me. )
They send brave messengers over the mountains and plead with the Morancians for help, so they come and join forces to drive the evil out. Then all the Great Forest villages agree to become part of Morancis. They build a good High Way through the mountains, and live happily and peacefully for a long time.
After many years, history repeats itself. The (same?) evil threatens Morancis. So they send messengers across the sea to a friendly trading country: Agrotha. The Agrothians come and help to drive the evil out, and then the Moracians agree to become a tributary to Agrotha. The Agrothians send a member of their royal family and his household to live in a castle in Morancis, to make sure everything goes as it is supposed to.
The Agrothian prince proves to be rather tyrannical, and does several things that make the people hate him. Under a leader, they arm themselves, gather a huge force, and surround the castle. The Agrothians are unable to send for help, so Agrotha does not know what is going on. The Moracians demand stuff from the Agrothian prince, but he refuses. After two weeks of this, the Moracians storm the castle.
They kill every single Agrothian inside (except one who escapes). Women, children, princes and servants. They don't stop killing until no Agrothian is alive inside the castle.
After some time, the king of Agrotha hears of this outrage. He sends his huge and powerful navy across the sea, and they raid the most important port city, killing indiscriminately. But the king is after revenge, not conquest, so after the city is flattened and burned, he retreats back across the sea, cutting off all ties, communication, and trade with Morancis.
The Morancians seethe under this insult. But the country is not yet strong enough for a counter attack, so, for nine more years, they try to set up a new government and build up an army and navy. Hatred of the Agrothians runs strong and deep.

Ask all the questions you want. I still need ideas.

A very good and realistic premise. My one question at this point... once the forest villages align themselves with Morancis, do they all act as one? So when the trouble with Agrotha arises, the villages also become tributary, and so forth. Right?

Yes...Right.
I guess the more vague I am, the less there is to find fault with, huh?

True.

Some people like to draw up very detailed histories. I am not that way. I know that sometimes detailed histories can be good for your story... but I'm more present-grounded myself. If it were me, this level of detail would definitely be enough to build the present setting and write the book.

Yes, I agree. I can discover the details as I go.