A kid lives on a farm on the border with a country that's going t o attack his country. He fights a guerrilla war inside the enemy country and defeats them with the support of his home country.
After the war he builds a harem which includes his sister.
Science fiction.
Farming
Did he have an eagle and play soccer? The first thing that popped to mind is Behemoth by FantasyLover.
Good Hunting, limab.
Agree it sounds like this one, though I think it's one of the few from the author where the MC doesn't end up with his sister in the harem!
I don't think that's the one that started the thread was talking about, but you're referring to the "Flight of the Code Monkey" by Kid Wigger https://storiesonline.net/s/74251/flight-of-the-code-monkey
It sorta had the homeland underdog, threesome thing with is sister going on but not much else.