I'm not going to debate the merits of the military conflicts you mentioned, as that is a red herring.  The question is whether he would require a country or a person to submit to deadly violence, even to the point of death, rather than kill an unjust aggressor in self-defense.  It's fairly clear that he would not.  He even told the apostles to go arm themselves with swords.  Do you find his words and counsel unconvincing, even though it squares with human nature and reason to defend one's life against attack?   

So yes, Jesus would allow a nation to protect itself by having an army.  NOW your questions become hugely relevant.  "What kind of army would Jesus form and lead? What kind of military action would He undertake?"  THOSE questions --- not the question whether he would countenance nations having armies; of course he would -- are the questions that count.