Sure - sorry this took me a bit to get back to - I was trying to avoid making load-issues worse.

I'm going to try and frame this in as charitable a ways I can to the information I'm providing. We'll see if I succeed. In effect, there are a number of different schools of theory in international relations, but it probably makes more sense to structure most of them as a response to the first formal theory to emerge: realism.

The core driving reality of realism is what it calls "international anarchy" (this is incorrect. What it actually means is international autarky). In effect, that means "there's no guarantor of state violence to curb behavior". The effect, therefore, is that there is no binding law in international relations. Consequentially, the only determinant of whether a given actor does something is their comparative capacity to enact violence. In the language of Thucydides: "The strong do what they will and the weak suffer what they must."

Realism draws another couple conclusions about states that are somewhat less well-supported, but still stem from the principle of international autarky. They are that every state is a rational actor (the schniff you just heard in the background was just my pet raccoon digging through the trash-can for some reason) within the framework of autarky and that, correspondingly, domestic political matters are largely not dispositive to international behaviors (wow, that little guy really can dig...). From this, realists develop ties to game theory as a predictive model for state behaviors, and they develop a lot of involved theories about exactly when states are most likely to use violence. There's a lot of potential topics here, but the only one I'm gonna give much attention is that they effectively provide the idea that there are three "states" that the international order can be in: multipolarity, bipolarity, and unipolarity. In plain words, that's "there are more than two great powers", "there are two great powers", and "there is one great power". This is their means of describing the history of the 20th century: it starts in the most unstable state, multipolarity, at the end of World War 2, it enters the arguably most stable state, bipolarity, and at the end of the Cold War, enters unipolarity.

I'm gonna hand-wave at the next, because you've lived in its ideological context your entire life. International liberalism posits that a combination of interdependent trade, international institutionalism, and greater communication between states can overcome international autarky. It starts out in the form of Woodrow Wilson's Idealist Liberalism, which just creates the Second World War, and then splits off into smaller theoretical cliques desperately trying to find the formula to overcome international autarky. I struggle to be particularly charitable, because we see how very rarely liberal internationalism actually predicts state behaviors.

Then we get to the more heterodox schools of international relations: structuralism and constructivism.

Structuralism doesn't really require much introduction, because it is us. Structuralism is the modeling of international behaviors based on material circumstances, and largely just encompasses marxism, but does have some other heterodox frameworks out there that I am entirely unfamiliar with. Apparently the term has gone out of use since I left academics, and now folks just explicitly say marxism again, which is probably better.

Constructivism effectively says that you can't just treat states as if they actually know the truth. You have to approach the world from the understanding that states have ideological lenses and they see the world through them. States view themselves as belonging to categories and groups and that biases their actions. But they also can misunderstand each other. Because the inherent truth of international relations is incomplete information (I know what you say, but I can't know if you mean it), states can effectively create conditions that place them into conflict needlessly. One of the most commonly assigned introductions to constructivism is entitled "International Anarchy is What You Make of It".

Constructivism serves to counterbalance realism's tendency to mistake states for having any idea what they are actually doing. In understanding the current situation in Ukraine, we can see that most NATO powers probably thought they could intimidate Putin into not taking action - that he didn't mean it when he made the demands he made. Putin believes that NATO won't do anything that does enough harm for him to not address the immediate threat of NATO at his border, and we're going to find out if he's right over the next few days. Ukraine, though, ended up getting fucked by this the hardest: When the first started seeing memos saying Russia would invade, they thought the US was just posturing to justify policies it wanted to pursue. As a result, they played up that threat to get more military equipment from the US. A few weeks ago, they came to the horrified realization that the United States had been completely serious, and that all they'd done is make Putin more worried by playing things up and expanding their armed forces.