Realist theories share the following key assumptions:

• The international system is anarchic. There is no authority above states capable of regulating their interactions; states must arrive at relations with other states on their own, rather than it being dictated to them by some higher controlling entity (that is, no true authoritative world government exists).

• Sovereign states are the principal actors in the international system. International institutions, non-governmental organizations, multinational corporations and other sub-state or trans-state actors are viewed as having little independent influence.

• States are rational unitary actors each moving towards their own national interest. There is a general distrust of long-term cooperation or alliance.

• The overriding 'national interest' of each state is its national security and survival.

• In pursuit of national security, states strive to amass resources.

• Relations between states are determined by their comparative level of power derived primarily from their military and economic capabilities.

In summary, realists believe that mankind is not inherently benevolent but rather self-centered and competitive. This Hobbesian perspective contrasts with the approach of liberalism to international relations which views human nature as selfish and conflictual unless given appropriate conditions under which to cooperate. Further, they believe that states are inherently aggressive (offensive realism) and/or obsessed with security (defensive realism); and that territorial expansion is only constrained by opposing power(s). This aggressive build-up, however, leads to a security dilemma where increasing one's own security can bring along greater instability as the opponent(s) builds up its own arms. Thus, security is a zero-sum game where only relative gains can be made.

While Realism as a formal discipline in international relations did not arrive until World War II, its primary assumptions have been expressed in earlier writings:

• Sun Tzu (or Sunzi), an ancient Chinese military strategist who wrote the The Art of War.

• Thucydides, an ancient Greek historian who wrote the The History of the Peloponnesian War and is also cited as an intellectual forebearer of realpolitik.

• Chanakya (or Chansh) early Indian statesman, and writer on the Arthashastra.

• Han Feizi, Chinese scholar who theorised Legalism (or Legism) and who served in the court of the King of Qin - later unifier of China ending the Warring States Period. His writings include The Two Handles (about punishments and rewards as tools of governance). He theorised about a neutral, manipulative ruler who would act as Head of State while secretly controlling the executive through his ministers - the ones to take real responsibility for any policy.

• Niccolò Machiavelli, a Florentine political philosopher, who wrote The Prince in which he held that the sole aim of a prince (politician) was to seek power, regardless of religious or ethical considerations.

• Thomas Hobbes, an English philosopher who wrote Leviathan in which he stated that in anarchy there is a "war of all against all".

• Otto von Bismarck, a Prussian statesman coined the term balance of power. Balancing power meant keeping the peace and careful realpolitik practitioners tried to avoid arms races.

• Carl von Clausewitz was a 19th century Prussian general and military theorist who wrote On War.