Realism is one of the oldest and most important theoretical approaches to IR. According to this theory, states are the only important actors in international relations. These states always act in ways that are meant to increase their own military power, or at least to prevent any other country from making significant gains in power relative to them. States have permanent interests that are not affected by their cultures, ideologies, religions, or anything else. The basic ideas of realism, then, are that states are the only actors that we need to be concerned with and that they want to maintain or increase their military power.
The major difference between classical and structural realism concerns the reason why states want military power. According to classical realism, states want power because of human nature. Human beings naturally want power for themselves. Since states are just collections of human beings, they too want power. Structural realism, by contrast, says that states want power because of the anarchical international structure in which they exist. There is no world government that can protect a state. Each state must protect itself as best it can. In response to this imperative, states seek military power.
Liberalism sees the world differently. It argues that different states have different goals. It argues that there are different kinds of power other than just military power. It claims that states are likely to cooperate with one another because war is too dangerous to be a good alternative. This cooperation leads to rules that govern the international system, making it much less anarchic than realists think.
Rationalism, or the English school, is seen by some as a “via media” or middle way between realism and liberalism. Unlike idealists, this school does not think that institutions can be created that will make rules for the whole world. However, unlike realists, they do not think that the international system is completely anarchic. Instead, they say that ideas and ideologies are important. States that share these will naturally gravitate towards one another and will tend to cooperate rather than compete for military power. In this way, rationalism can be seen as a middle way that borrows important ideas from both liberalism and realism but does not go as far as either.