Why do historians refer to the period between World War I and World War II as the "Age of Anxiety?"

Paul Tillich, a theologian, coined the term "Age of Anxiety" in the mid 20th century to refer to the sense of meaninglessness, particularly in Europe, that developed after World War I. This war, fought between great empires such as the Austro-Hungarian Empire, Czarist Russia, and Imperial Germany (among others), saw these empires crumble by the war's end. In addition, the types of destruction unleashed by the war were new, such as the use of poison gas, and these weapons and violence also targeted civilians. After the war, people were faced with the destructive results of the war and with the political aftermath of the war. Germany had to pay large reparations that hobbled its economy and helped pave for the unfortunate rise of Hitler and the Nazis, and Russia embarked on a Bolshevik Revolution and Civil War. 

Some of the sense of anxiety in the inter-war period arose from Europe's instability after World War I, when so many old Empires were broken up without stable new systems in their place. In addition, on a philosophical level, people were faced with new forms of modernity, such as the changing role of women and the sense of meaningless that resulted from the war, that left them feeling uneasy. While the old forms of government and society had been broken down, the new forms were too new to be stable and accepted. After the Great Depression started in 1929, people were further destabilized, leading to the rise of Fascism and to the beginnings of World War II.