From what I've heard Iran and North Korea can be taken out with the forces and technology we have revealed today. The problem with N. Korea isn't the war. The problem is the peace which comes afterward. It would be expensive since they have no viable means of income to spend in rebuilding the infrastructure. Iran is different. We can attack Iran and they have the means to rebuild. There is an ability to handle NK without going to war by raising the level of Japan and S. Korea capability to a point where it effects China and Russia. Both realize this and don't want 2 addition threats rising in the East. Iran is different we could have a war with Iran
Russia and China is a different story. The how and with what we would fight would depend on a bunch of variables.
I'm one of those people who believes the USA has at least 2 levels of ability ( maybe more ) between conventional and nuclear and another level over nuclear which are what I term technology levels. These levels differ by levels of technology. Other countries have some of these. But no one has the technology levels of the USA. No one can win a war mano y mano against the USA. China and Russia together could win a present technology fought war against the West. But if this occurs one can be sure the West will rise to higher technology levels.