- Sovereign countries have agency. Sure, countries may pay costs for disagreeing with the most powerful actor, but they have the ability to decide. So, you can't just do X and expect everyone to go along, whether they are friends or foes. Indeed, every country has its own domestic politics and its own national interests. Sucks, but there it is.
- Which leads to: cooperation is not easy but generally required. For instance, unilateral sanctions do not work unless one is the only supplier or buyer of a good/service. The US sanctioning Iran can do some modest damage, but to really punish Iran, the US will need Europe and probably Russia and China to get on board. Why should they? Wishful thinking?
- Strategic thinking is key. To be clear, this means that one needs to figure out not just what one wants, but what others want, so that one can figure out ways to get others to do what you want. My best illustration for this was always my dogs--who remain more strategic than Trump despite both dogs being dead for seven and ten years respectively.
- The security dilemma is a thing: unilateral efforts to improve one's security will cause others to react, improve their defenses, which ultimately leaves every less secure. So, yeah, Iran pursuing nukes may very well lead to Saudi Arabia pursuing nukes. Good times.
- Which leads to this: repeatedly issuing threats is a bad idea. We IR folks tend to teach WWI for many reasons, but one of them is that the Germans thought they could cow everyone into bending to them. Instead, they provoked alliances against them. Trump thinks he got denuclearization in Korea (nope, he didn't) because he threatened a lot, so now he is going to apply "maximum pressure" everywhere. This will only come back to bite in a big way. I won't get into the details of Prisoner's Dilemma vs. Chicken here, but the basic idea is that if you keep on playing chicken, at some point, someone is going to escalate and it won't end well.
One last thing: as old IR colleague of mine commented on my fb page: individuals matter. Trump reminds us of this everyday. Not all Intro to IR classes take individuals seriously. I had a lecture or two on individuals, but spent most time talking about the importance of domestic politics and the dynamics of the international system. Why? Because the "great man" view of IR, including its inherent sexism, is what students get before they show up at college. But if I were teaching Intro to IR these days, I would probably spend a smidge more time... not just because of Trump but also because folks are thinking more seriously about it (we did in our NATO book).