Besides Realism, Liberalism is one of the greatest schools of knowledge in International Relations. Just like Realism, it is not easy to define Liberalism, for liberals come in many shapes and colors. However, I believe we can point to some core characteristics of liberals in International Relations.
One of the difficulties we find when discussing liberalism in International Relations is the same difficulty we have with Liberalism in general. Different from Marxism, for example, Liberalism is a very broad intellectual tradition, with many different thinkers. Sometimes I ask my students “who is the most important Marxist thinker?”. I hope they will answer Marx! And then I ask “who is the most important liberal thinker?”. Besides that, Liberalism went through a major transformation between the 19th and 20th centuries. One of the ways to make a distinction between the old and the new liberalism is to talk of classical liberalism and modern liberalism. Classical liberalism is very similar to what we call conservatism (or even Realism!). Modern Liberalism is often associated with the Democratic Party in the US.
In any case, I believe that the central tenet of liberalism is the defense of liberty. Liberals (especially classical liberals) believe that if individuals are set free from outside constraints, the natural result is progress. In other words, Liberals have great faith in the possibility of change – positive change. This contrasts with the general pessimism of Realists.
In very practical terms, although they agree with Realists that the International System is anarchic, Liberals see more space for cooperation between states towards a more pacific and prosperous World. Where Realists see competition, Liberals see at least the potential for cooperation. One of the ways that states can cooperate with one another is through shared values. These values can be fleshed into international organizations, such as the UN or the WTO.
In sum, liberals agree on a lot with Realists but have much more hope for international cooperation. I must say that I really want them to be right, but think that they are wrong. Realists seem to have a very strong point when they show how much the anarchy in the international system stops greater cooperation. And Liberals themselves are not waiting for a World government that will somehow solve that. I’m not saying that cooperation and progress are impossible or that they are undesirable. I’m just saying that I’m not convinced that they can happen the way Liberal Theory of International Relations describes.