The terms realism and liberalism are used to describe two opposing views about international relations. Realism claims to make an honest assessment of the way states relate to each other. It does this to the extent that war and conflict are understood as common tools of conflict arbitration. Liberalism, or idealism, in international relations is more interested in the way states should relate to each other to secure the best possible outcome for all people. Both traditionally view the state as the primary actor in international politics.

Descriptive Realism

Realism aiming to describe how states interact is descriptive. It sees self-interest and competition at the heart of international disputes -- and holds that states are motivated by desire for power. While it does not completely dispense with morality, it sees morality as a minor motivating factor that is largely unhelpful in diplomacy. War is not particularly unusual because the international system is defined by chaos and anarchy. Efforts to establish international peace are most effective, descriptive realists say, when they address not morality but what states gain from peace.

Prescriptive Realism

Prescriptive realism, developed in the 20th century by scholars including Kenneth Waltz, encourages the use of empirical data in the form of statistics. It is sometimes called neorealism or scientific realism. The idea is that large data-driven studies may be helpful in predicting future wars. Unlike descriptive realism, prescriptive realism sees itself as more scientific than theoretical -- and sees moral concerns as irrelevant to state actors. Unlike descriptive realism, it holds that states are primarily motivated by the need for security, not power.

Classical Liberal Universalism

In international relations, classical liberalism applies universal moral principles to all people because they are human. It sees the objective of international diplomacy not as establishing power or security, but basic liberty for all. Like realism, classical liberalism -- as illustrated by German Enlightenment philosopher Immanuel Kant -- posits the state as the primary actor. Though Kant himself opposed imperial aggression, other liberals such as John Stuart Mill have understood lack of basic liberty and rights as a justification for another state to invade and impose liberal values by force.

Fracturing in Liberal International Relations

There is disagreement in contemporary international relations about the extent to which universal values should be imposed by force. Human rights advocates including Anne-Marie Slaughter and Samantha Power advocate for broad interventionism in developing countries to protect human rights. Others, such as the late John Rawls, argued for a measure of tolerance for illiberal societies that do not terrorize their citizens. Other liberal international relations scholars including Thomas Pogge have focused on the argument that welfare state policies should be applied globally to ensure an adequate standard of living for all.