Yes, Halacha and the Torah deal with ethics at the national level. Some Rabbis, such as Rav Kook and ilk, even argue that halacha is mostly concerned with the ethics at a national level. They focus on the Klal and the Tzibur as the main ethical imperative, using individuals as the mechanism to achieve that moral dimension.
The Torah, and Jewish history teaches a policy of military isolationism in regards to moral failures of other states.
Although Israel is commanded to wipe out the 7 nations of Cannan for their moral failures, and the land is to expunge them, Israel is not commanded to destroy any nations outside of it's borders, save Amalek. Amalek, is given special treatment because of what they did to Israel on their way out from Egypt.
Despite the fact that neighboring countries to Israel were practicing pagan sacrifices and passing their children through fire, Israel was never commanded to war with them so that they would stop these practices.
Further, during the time closer to Roman and Greek era, Israel's fighters were often invited to neighboring countries armies to help them fight some other force. However, Israel never did this on their own, and only fought defensive wars, offensive wars for the land of Israel, or joined a war when invited. Similarly, other nations would only help Israel if they were asked, even when "treaties" were signed of mutual defense. That is, the Spartans did not help Israel until after Israel asked them to, with a reminder of the mutual defense pact.
The Jewish people are taught to lead by example, not by bashing and attacking people who fail to live up to the standards we expect from them. The Talmud also teaches to push with the right hand, but pull in with the left. By all definitions of the term, war is never a force which "pulls with the left", but rather going to war is like pushing with both hands. Sometimes that is needed, such as when emptying the land of Idol worship, or defending your land, but that is not the way that the Torah teaches us to influence other nations.
This is all implied by Rambam in his Halachot of Wars and kings, when in Halacha 1 chapter 5 he writes:
Afterwards, he may wage a milchemet hareshut, i.e. a war fought with other nations in order to expand the borders of Israel or magnify its greatness and reputation.
However, when it comes to non-war activities, the Prophets teach us a different lesson. Yonah was sent by Gd to go to Nineveh to warn the Assyrians that they need to repent else face the city being overturned. It is clear from here, that the Jewish people do need to help those of other nations, but should do so peacefully and with words, not with military action. Trade embargoes, harsh words, aiding the enemy of the evil nation, are all valid and peaceful ways of "intervening" with nations that behave wrongly.
As to your question about North Korea, I was actually thinking about that today while listening to the news about Syria. Why doesn't Israel intervene and become the "hero" in this dire situation?
I think the answer to this is the same as the answer above. If the people/leadership of the "rebellious" faction. (That which is being killed by their government) does not ask for intervention from a specific country, then said country does not have the permission to get involved. However, based on the last line of the Rambam, it seems that if saving the people from their government would increase the reputation of the nation doing so, then it would certainly be allowed. As we saw in Iraq however, saving a people from their oppressive government is not always an enhancement of reputation. Yet in Libya or Kosovo it was. According to Rav Kook's perspective, you can see this sort of logic played out on an individual level with regards to the theoretical halachas of who you are allowed to save on Shabbat and other moral dilemmas, even though in practice an individual should save anyone. (For various different theoretical reasons). To be more clear, even though the practical halacha of an individual and the nation are different, the theoretical constructs behind the situations are the same.