Well war simulator is kind of confusing. Civilization could fill that but I guess is not what you are looking.
Total War Games may be what you want. You control a, lets call it "state" (province, country, clan, depends on the game) and you'll be fighting other states in a map a making all kind of decisions and also have the actual battle that is played on real time.
My favorite is Shogun 2 (because weeb) but Empire and or Napoleon total war are very good.
total War Three Kingdoms is based on china. Haven't played it.
The newest one are the Total War Warhammer series (3 of them). Have heard great things about them but haven't played them either.
But all of these are not on modern era (and the warhammer is straight up fantasy).
If you want to control the politics side and the strategy in combat situation I can't think on anything better.
Heart of Iron has, as mentioned by others, an steep learning curve. Its a whats called a grand strategy game so you won't be controlling units like in a rts.
Similar to HoI, If you don't mind the middle ages, Crusader Kings 3 can be epic, nerve wrecking and hilarious at times. In it you roleplay a person, leader of a family/clan and make the decisions that will make the "defend myself or attack other countries etc " you describe. Also, your counselors may not like you and throw you in jail, so there is that.
I still remember the legend of Usur Ibn-al-jihar. His father sucked much ass but he manage to recover what his father loose and then some and brought to his knees two spanish queens. And the king that remained on the other limit of his lands were probably trembling when I stopped playing that time.