To contrast these two approaches to foreign policy, it is perhaps easiest to use realism as a baseline. Realism in foreign policy terms is based on the idea that all states must pursue a policy of self-preservation above all else. While there are differing strains of realism, the general thrust of this way of thinking is that each state must act essentially on its own and only in its own interests. To put it in personal terms, no state can trust any other state, according to realists. Taken to its extreme, realism basically assumes that international relations is a zero-sum game and really a state of anarchy in which power is the sole determinant of affairs. Realists interpreted the Cold War, for example, as a system in which global stability existed because of the power exerted by the United States and the Soviet Union.
Liberalism operates on a different set of assumptions than realism. Where realists see states as basically monolithic entities focused on self-preservation, liberals tend to emphasize the ideologies that form the basis of each state's government. These ideologies play a role in understanding the foreign policy decisions each state makes and sometimes lead to a different set of policy prescriptions than realism. For example, while liberals accept that security is an important goal of foreign policy, many liberal theorists have argued that democratic countries do not go to war against each other. Therefore it makes sense that the pursuit of security entails promoting democracy around the world. Liberals also emphasize economic motives, which, like the rights central to a democratic society, are individual (or corporate) more than state-centered, as an end of foreign policy. This means that they may push for international cooperation, a goal not necessarily coherent with a realist worldview.