A Reductio ad Germania for all of the First World War's causes has been a popular one almost from the start of the war. It's one that started as an act of propaganda that started as far back as the fall of 1914, mostly in the British and French press, and it only grew in popularity through the course of the war as they vilified the German occupation of Belgium and northern France. This popular perception was then all but enshrined in the various articles of the treaties that came out of the Paris Peace Conference. While the Treaty of Versailles is the most famous of the treaties, and it is the treaty that is generally believed to be the one that ended the First World War, each of the belligerent Central Powers signed their own separate treaties. And each and all of these treaties had what was considered in later years to be a guilt clause that helped create a sense of responsibility on the Central Powers for those unfamiliar with the nuances of the article in question.
Each of these treaties that came out of the Paris Peace Conference, between January 1919 and June 1919, the Treaty of Versailles (which dealt with Germany), the Treaties of Saint-Germain-en-Laye (Austria), of Trianon (Hungary), of Sévres (The Ottoman Empire), and of Neuilly-sur-Seine (Bulgaria), were all very similar, with regards to their structure and format, as well as their intent. On the whole these treaties were designed to do the same thing for all of the defeated belligerents: to reduce their territorial extents, to limit their warmaking capacities, and to ensure that reparations and other indemnities were made to the victorious parties, while also refuting any future claim to the redrawn boundaries. Reading through the the five treaties, one will see nearly identical language and structure. Included in these similarities is the so-called War Guilt Article, though it was never called as such at the time, nor intended to be framed as such, at least as it was written. This article (231 in Versailles, Article 175 in Saint-Germain-en-Laye, Article 120 in Neuilly, 161 in Trianon) simply laid the legal framework for the reparations scheme that the Entente had agreed to amongst themselves.
It reads as follows (I quote from Article 231 in Versailles, but the wording is identical, with the appropriate national substitutions being made for the other treaties):
"The Allied and Associated Governments affirm and Germany accepts the responsibility of Germany and her allies for causing all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by the aggression of Germany and her allies."
This wording, written by Norman Davis and John Foster Dulles (in 1919, the American delegation's legal council and an aid to Robert Lansing, the American Secretary of State, and Dulles's uncle), was carefully phrase in such a way that there was no actual declaration of guilt in this article that is generally perceived to be the War Guilt Clause. Instead, Article 231 and its ilk were carefully crafted in such a way that the various belligerents simply acknowledged their culpability in the damages caused by the war, as the loser of the war. This is not a statement of guilt, as written and phrased (though Dulles would later write of a certain degree of guilt surrounding the clause, this was done much later after the longer-term consequences of it would be more apparent). Instead, it is an acknowledgement, that lays the legal foundation for the claiming of reparations. Without establishing responsibility for destruction, these reparations would be functionally unenforceable. These sorts of articles were part and parcel to countless peace treaties.
These articles were neither new, nor particularly harsh. And on the whole, with the exception of Germany, there was no wild backlash, no outrage, no...nothing, really, surrounding the reparations articles in the associated treaties. These largely went unremarked-upon, and most of the popular and national backlash in those countries surrounded many of the other articles, such as the military restrictions, or the territorial reductions, population transfers in line with the new political borders, and other associated matters. It was only in Germany that Article 231 really became a hotbed of resentment in and of itself.
Now, the reason for this perception of Germany being the cause is several-fold. Firstly, there is some truth that Germany pushed for war. There is truth too in the fact that the Blank Check, laid out by the German government to the Austro-Hungarian government as it was drafting the ultimatum to Serbia in July 1914 likely gave Austria the political stiffening required to see that it went through with a much more hardline approach on Serbia. It was also true that there was an acknowledgement in the German military that if war was to come, the sooner it happened, the better, before Russian population growth and military-industrial modernization (begun in 1912) would overwhelm Germany, especially as she was simultaneously squeezed from the west by France, and in the east by Russia, and that projections suggested that by 1916 or 1917, there was no hope for a German victory if a war were to start at that time, or later on.
However, beyond that, it is largely acts of British propaganda as it rallied around "Gallant Little Belgium" as it was overrun by the German advance, that captured the public's attention and began to start solidifying Germany as the root cause of the war. It is also acts of French propaganda as the Germans occupied vast swathes of territory in northern France, and that the French government began to see the war as an opportunity to reclaim what was lost in 1871, that Germany quickly became the primary villain of the war. This was not helped at all by the fact that except for a few token units in the south, near Switzerland, there were very few Austro-Hungarian soldiers in the West, and German forces fought not only in the West, but also in the East along Germany's own border with Russia, but also were extended significantly into Austria-Hungary's border with Russia as Germany was forced to give aid to the collapsing Austrian army. Further, more German forces were sent into Serbia as well to aid there. Thus, while Austria-Hungary may have been initially responsible for the war, it proved itself to be spent offensively in mere weeks, and was on the back foot in the span of months. An enemy that can't even be called a paper tiger is not a very good enemy to drum up propaganda against. Least of all when the Anglo-French alliance was fighting in the West primarily, and against the Germans, and not the Austrians. The Austrians were simply not compelling enemies in a lot of ways during the war from a propagandistic perspective. Further, the initial declarations of war by Britain and France were against Germany alone.
Britain and France did not declarer war on Austria-Hungary until August 12. In contrast, the French declaration of war against the Germans was announced 3 August; and the British declared on Germany on 4 August with the violation of Belgian neutrality. As such, in the Anglo-French perspective, which is what drove much of the post-war perception due to Russia succumbing to revolution in 1917 and civil war thereafter, and a general Western European bias in history to begin with, the First World War was a war against Germany.
In short, German guilt for the Firs World War neatly fits into the popular expectations of the period, fitting with the lived experiences, as the death of Franz Ferdinand in June 1914 was a tragedy, but barely went remarked on, and the July Crisis was something that was likewise considered to be a thing that would soon pass, as these things tended to do, with a bit of saber rattling before the world went back to normal. War, if it happened, would be a localized affair, and just another conflict in the Balkans. Remember, the Balkans had been perennially at war since 1911 and these kind of conflicts had become routine. It was only when German troops crossed the borders in the West that things "got real" for a lot of people, and when it is German, not Austrian, troops doing the crossing, and it is these events in the West that suddenly capture the rapt attentions, that German guilt can start to be appreciated as the perception in the time period.
Additionally, this view of what I termed at the opening reductio ad Germania is not just an act of popular perception, but also very real political perception, primarily in France and Britain. I touched earlier on how Dulles and Davies had written the final wording. This wording comes from a stern compromise between American views on wanting no reparations, and an Anglo-French view that Germany should pay for *all* war costs, not just civilian damages. This view stemmed largely from that popular sentiment and outlook on responsibility. These French and British politicians had genuine concerns on their political futures if they did not sufficiently lay the blame at Germany's feet after such a costly war, because it was the Germans that had been the enemy in the popular imagination.