@Leo_TheSilent007 From what I saw, his country is poor and weak, but stands between the mountain border to the next nation. The Empire wants to annex the next nation and liberate the enslaved people. To save troops, they will not fight his country, and will train them to be the defense/supply line in case the Empire loses the war. If the Empire wins the war, they will gain a vassal state to exploit. However, if his country gets strong enough, they can wait until both armies are exhausted and take over the other nations.
Tl;dr, it's a waiting game for who loses their military force first.