First paragraph- all true. But we also have enough nuclear weapons to destroy Russia in an hour as well. And they know it. Which is why there are far worse situations than Mutually Assured Destruction for keeping stability in place.
North Korea can't reach the US? That is by no means a certainty. They did just launch a "satellite" carrying rocket which has a range of roughly 10,000 KM, which would put it roughly in the LA area if fired at the US. The quality is, of course, unknown, as is their ability to replicate the feat. They have had far more failures than successes; that said, the main concern I see is the rate at which their weapons program has been developing. We are no longer talking decades between their development of new missiles, but years instead. And keep in mind, they don't need a whole lot of range to hit a major US ally as well (Seoul or Tokyo). To boot, they also have ties to both Russia and China, which isn't the best situation for U.S. interests either.
Russia is allied with Iran, true. And they are fighting in Syria, though Syria is a difficult situation since, while they are backing Assad, it's not like most of the groups fighting Assad are good guys either. Difference i see there is that while Putin is the pure emodiment of the former KGB officer that he is, and is well practiced in lying his ass off, the fact remains that Russia has a great deal of interest in continuing normal relations with the rest of the industrialized world. So while they may flex their muscles here and there, they are NOT going to unilaterally start a major war with the US or one of our major allies. Whereas I can easily picture Kim Jung Un, in a fit of psychosis, deciding to fire at Seoul or Tokyo, or even taking a chance at Paris, London, Hawaii, or even the West Coast of the US. Granted, their chances of actually managing to reach anything further than Tokyo aren't great right now, but a couple years down the line, that could very well change. They admit to having nukes, they pulled out of the Nuclear Non proliferation treaty years ago, they have cut ties with 95% of the world, and their populace is pretty much used to having nothing and thanking the Great Leader for providing it. Paranoia is rampant, the US is blamed for everything, they constantly proclaim that we are going to get involved in helping South Korea take over their country, and they have a gigantic military. And lacking a whole lot in the way of natural resources or manufacturing, they are a perfect recipe for starting a war somewhere to gain land area and those resources.
All three of these countries pose a threat. So does Pakistan, where the factions that control the military and their nuclear cache seem to change on a daily basis, and a number of the former Soviet states that still possess nuclear weapons in various states of decay. While the former Soviet states are not about to attack us themselves, they can sure as hell equip a state or non-state actor that will.
The world is pretty damn complex, and to simply write off North Korea is ridiculous. To say the Bernie Sanders is insane for being concerned about them is beyond ridiculous. I realize from your post history that you are not a fan of "leftist solutions", but the best solution in this situation is a purely leftist one: Find a way to make peace instead of more war. When Obama talks to Iran to come to an agreement about nukes, don't have the GOP congress send out a letter saying that any deal he comes to is null and void in 11 months. Don't politicize the possibility of death on a massive scale. Obviously the Republicans missed that lesson.