I also don't understand why people are complaining about soo won acting weak and second guessing his plans. I think it is very realistic. His death is getting closer and closer, you can't expect him to behave in the same way as when he was considerable healthy and still has time. When you've lived your whole life working towards something but then your time keeps getting shorter and shorter to fulfill it, of course you'll become desperate and start looking for alternatives. If you expect a character to stay one dimensional and not change according the situation they are in, then you are expecting a terrible character.
Ikr! He's quite a complex character and I like him because of that.. (〜￣△￣)〜
People here really think that if you just you the "right thing" things will work out. When that is literally not how anything works in the real world. You have to make hard choices and you change your strategy according to how the current situation is. When you are at a constant threat of being invaded, It is either kill or be killed. There is no right or wrong in that situation. Expansionist is someone who conquers other territories even if they don't pose a threat and will not stop at that. Protecting your kingdom is not eXpANSionisT.
Also, it is very much possible that soo won is not considering Yona because he doesn't want her to bogged down under the weight of the throne where you are faced with a constant threat to life. I won't say I agree with this choices, but I understand them. You can be morally wrong, but still do the right thing for the greater good.