the maintenance of a balance of power could in some circumstances require a willingness to go to war, some critics saw the idea of a balance of power as promoting war rather than promoting peace. This was a radical critique of those supporters of the Allied and Associated Powers who justified entry into World War I on the grounds that it was necessary to preserve the balance of power in Europe from a German bid for hegemony. In the second half of the 20th century, and especially during the cold war, a particular form of balance of power – mutual nuclear deterrence – emerged as a widely held doctrine on the key to peace between the great powers. Critics argued that the development of nuclear stockpiles increased the chances of war rather than peace, and that the "nuclear umbrella" made it "safe" for smaller wars (e.g. the Vietnam war and the Soviet invasion of Czechoslovakia to end the Prague Spring), so making such wars more likely. Free trade and interdependence It was a central tenet of classical liberalism, for example among English liberal thinkers of the late 19th and early 20th century, that free trade promoted peace. For example, the Cambridge economist John Maynard Keynes (1883–1946) said that he was "brought up" on this idea and held it unquestioned until at least the 1920s. During the economic globalization in the decades leading up to World War I, writers such as Norman Angell argued that the growth of economic interdependence between the great powers made war between them futile and therefore unlikely. He made this argument in 1913. A year later Europe's economically interconnected states were embroiled in what would later become known as the First World War. Democratic peace theory The democratic peace theory posits that democracy causes peace because of the accountability, institutions, values, and norms of democratic countries. Territorial peace theory The territorial peace theory posits that peace causes democracy because territorial wars between neighbor countries lead to authoritarian attitudes and disregard for democratic values. This theory is supported by historical studies showing that countries rarely become democratic until after their borders have been settled by territorial peace with neighbor countries. War game The Peace and War Game is an approach in game theory to understand the relationship between peace and conflicts. The iterated game hypotheses was originally used by academic groups and computer simulations to study possible