Short answers, pulled from the story.
The Second Thirty Years' War is a periodization scheme that treats the European conflicts from 1914 to 1945 as a single continuous war rather than two separate world wars. The thesis holds that World War I naturally led to World War II, with the interwar period serving as a truce rather than a true peace.
Charles de Gaulle originated the concept in a speech at Bar-le-Duc on the 28th of July 1946, invoking "the drama of the Thirty Years War we just won." Winston Churchill reinforced the idea in 1948 in the preface to The Gathering Storm, writing that his books would cover an account of another Thirty Years War.
The framework spans major European conflicts from the Balkan Wars (1912-13) through World War II (1939-45), including World War I (1914-18), the Russian Civil War (1917-23), the Ukrainian-Soviet War (1917-21), the Polish-Soviet War (1919-21), and the Spanish Civil War (1936-39). The interwar years also saw significant civilian, labor, and colonial conflicts.
Sigmund Neumann, in The Future in Perspective (1946), divided the period into five acts of approximately equal length: 1919-24, 1924-29, 1929-34, 1934-39, and 1939-45, likening the arc to a Greek drama with World War I and the Versailles peace as a prologue.
Critics argue the thesis is too simple and risks excusing Nazi historical actors by framing their rise as an inevitable reaction to the Treaty of Versailles. Because Hitler's rise depended on the Great Depression, scholars argue it was not predetermined, and the thesis cannot account for the specific choices made by fascist and Nazi leaders.
Both concepts view the 1914-1945 period as a single prolonged struggle within Europe rather than a sequence of distinct wars. The European Civil War framing emphasizes that the conflict was driven by the problem of Germany and by competing ideologies including fascism, Nazism, and communism that emerged after World War I.