The main thing that you learn in a course on International Relations is about the main theories of IR and how they attempt to explain the ways in which countries interact with one another.
There are three main theories in IR. In a nutshell, they are:
- Realism, which says that countries always act so as to preserve or increase their power. All they care about is power, not ideology or anything like that.
- Liberalism/idealism, which says that countries can help themselves by cooperating rather than competing. It believes that countries that are democratic won't fight each other and that bodies like the UN can bring about more cooperation and less conflict.
- Constructivism, which says that countries interact based on how they see themselves and one another. Countries won't just compete all the time the way the realists say they will. Instead, countries who see themselves as similar will cooperate, those that see themselves as different will compete.
Again, these are very much "nutshell" looks at these theories and there are other theories that some teachers will see as more important. However, when I teach IR, I teach mainly these theories and how they would interpret or explain various events in the world.