When the study of International Relations first began in modern times (1930s and forward), it was largely considered a theoretical discipline. The first two foundational volumes were, indeed, theoretically inclined: E.H. Carr’s The Twenty Years’ Crisis (1939) and Hans Morgenthau’s Politics Among Nations (1948). These were formative in the development of the theory of realism, as had been the prior works of Thucydides (The Peloponnesian Wars, in the mid-300s BCE), Thomas Hobbes (Leviathan, 1651) and Niccolò Machiavelli (The Prince, 1532).