Russia and China both have nuclear tipped missiles capable of hitting the United States, either from land based launch pads or warships or submarines. To our knowledge, they do not have sufficient landing craft or troop carriers to land an invasion force in a size sufficient to overcome the resistance they are likely to encounter if they attempted to actually invade the United States. Are they a threat to invade? The answer should be a resounding no for the very same reason Hitler did not invade Switzerland in World War II.
Hitler was successful in overtaking Austria and the Sudetenland early in his campaign because they in essence welcomed him without resistance. His invasion of Poland, the Low Countries of Holland and Belgium and even France was because they were unsuspecting, unprepared and the blitzkrieg tactic of massive force employment prevailed before his foe could mount any meaningful defense. It is the same tactic used by the U. S. and its allies in the Gulf war against Sadaam Hussain who was unsuspecting of the response and whose supply line was already stretched.
Russia and China know full well that any thought of invading the United States would be a suicide mission. They are also well aware that the use of nuclear weapons to obtain land is a non-starter because, if your objective to obtain territory, you gain nothing if it is contaminated by radiation and rendered useless for both habitation and food production. Russia is acutely aware of this fact after their experience with Chernobyl.
You have to be either insane or brainwashed to believe North Korea is a threat to the United States. Unfortunately, it is the latter and it has been so successful the government can now support its military industrial complex monopoly with nary a peep from the brainwashed electorate. That this is true not only for the military but the civilian side where after successfully landing on the moon we are now spending billions to attempt to colonize Mars that any rational person knows is impossible.
So, why is North Korea threatening the United States? The simple reason is they see the U. S. as a better benefactor of their economy than the Chinese, who prop it up to keep the North Korean surplus population in North Korea rather than have them migrate to China. If you examine the trade balance you will uncover the fact that China is actually importing food because it is also exporting it, to North Korea who does not have the agricultural capacity to feed its own people.
What would happen if we just ignored North Korea, the “Mouse That Roared”? They would have to continue to rely on China or invade South Korea in order to obtain the wherewithal for their survival. With a large troop commitment in South Korea we would have the choice of leaving them at the mercy of the North Koreans or as we are doing in Iran, Afghanistan and Syria continue to supply them with mercenaries. Which do you think we will do?