Quendi, I disagree that WWI was watershed event in the 20th century. Only one battle of WWI is considered as top 25 battles of humanity by military analysts, and this is I. battle of Marne. No other battles of WWI achieved such prominence. WWII was a watershed of the 20th century. For many countries WWI was indirect. No city between Rhine and Carpathian range witnessed a war. War affected four areas of Europe like Belgium and France, Venetia, Balkans, and Baltics to Black sea region. Iberian and Scandinavian penninsulas were not affected by war. USA lost about 40,000 people in the war, much less than in Vietnam war. Political structure on local level survived the war, even in countries like Hungary, Poland, Czechoslovakia, and Austria. The rulling dynasties were gone, but much of the political, economic, and cultural system survived well until 1945.
WWI is seen as limit what is called a Long Century. It started with the defeat of Napoleon and establishment of Quadruple alliances, and survived with minor changes until 1914. WWI was also an answer on changed demographic in Europe especially in the Central Europe dominated by Vienna. Austria-Hungary was after Russia the biggest European country and second most populous until 1900 ahead of continental UK and France. At the turn of the century it was replaced by unified Germany. German influence within Austrian monarchy wanned from dominant force around 1850 into minority where only 1/4 of population considered themselves as a German speaker. Disintegration of Austrian monarchy was inevitable and the monarchy only held by strong persona of Franz Joseph who rule the monarchy for 68 years from 1848 till 1916. After his death, there was no any leader that was capable managing this state and by 1918 Germans were afraid of ethnic instability from Bohemia to Balkans as bigger threat than American entrance into war. In 1918 Germans defeated Russia and occupied most of Ukraine, Baltics, and Balkans; Italy was decimated, and they were standing deep in Belgian and French territory. WWI was pretty much stalemate and had not achieved its orginal goal. Germany did not occupy Paris, and French-British alliance did not inflict significant defeat to German's hegemony in Europe. This all happened 25 years later.