Honestly, when you get into politics as complicated as the brew-up that led to the First World War, it can be difficult to point to any one event, as well as defining 'cause' in the first place. We can certainly blame the spark that landed on tinder for starting the fire, but would it have been that bad had the tinder not been there? Would this discussion be different if the tinder were placed in a hot environment outputting a lot of sparks that could fall on it at any moment? And so on, and so on - and noting that accidental fires are still a lot less complicated than the mess that is international politics.

More can always be said on the matter if anyone else would like to tackle this most fraught of topics; for the meantime, u/Starwarsnerd222's six-post treatise on the 'MAIN' explanation as regards the causes of the Great War. Get yourself a drink before you go in, it's a chunky 'un.

Greetings! u/DanKensington's linkdrop to my (rather lengthy) Saturday Showcase on the matter should be illuminating enough to show just how hard it is to illuminate a singular cause of the First World War, but I wanted to touch a bit more on the boilerplate to Dan's comment, which reveals a fair bit as well about the issues surrounding the causes of the First World War. This rabbit hole has attracted (and helped make) so many famous names in historical academia: Fritz Fischer, AJP Taylor, David Stevenson, and Max Hastings to name a few. Yet all of them have explored it in different ways, each coming to different conclusions, from the sheer depth of evidence and primary source work that has been made available on the matter. If you were to ask this question in a hundred years, I doubt anyone on AH (or in the world for that matter) would be able to definitively explain the "cause" of the First World War.

I absolutely detest school curriculums and textbooks which claim that the assassination of Archduke Franz Ferdinand caused the First World War. The death of a single European statesman is nowhere near satisfactory enough to explain why all of the Great powers of the day (Britain, France, Austria-Hungary, Russia, and Germany) went to war for four long years. Certainly, the Archduke's death reignited (to use a bit of Dan's figurative language) longstanding tensions between Austria-Hungary and Serbia, but that does not help explain why the war was not simply a "Third Balkan War", localised to a single region of Europe which had been so unstable in the decades before 1914. To quote the Saturday Showcase:

"When shots rang out in Sarajevo and killed the heir apparent to the Austro-Hungarian throne, they rang out in a world beset with a litany of factors which, despite none of them being a key casus belli, presented the leaders of Europe with a geopolitical situation where much was at stake. The militaristic war plans and naval arms race were not turned to as the first option; though the decision to call forth the reserves and pack the troop trains was one which helped the dominoes to fall quicker than they might have had in previous years, but they alone cannot explain how an entire continent marched off to the front."

Margaret MacMillan, whose work on the leadup to the First World War I highly recommend, also echoes a similar stance in this quote from that work:

"There are so many questions and as many answers again. Perhaps the most we can hope for is to understand as best we can those individuals, who had to make the choices between war and peace, and their strengths and weaknesses, their loves, hatreds, and biases. To do that we must also understand their world, with its assumptions. We must remember, as the decision-makers did, what had happened before that last crisis of 1914 and what they had learned from the Moroccan crises, the Bosnian one, or the events of the First Balkan Wars. Europe’s very success in surviving those earlier crises paradoxically led to a dangerous complacency in the summer of 1914 that, yet again, solutions would be found at the last moment and the peace would be maintained."

One could approach the Origins of the First World War in so many different ways, each one as unique as the other. I myself continue to ascribe to the geopolitical causes of the First World War, the longstanding rivalries of Europe reignited over the assassination of an Archduke, prompting various desires and aims within each nation. Coupled with all of this, there is the whole business of putting the blame of the First World War, as no doubt mainstream (and even tertiary) sources will point towards Germany, Serbia, or even Austria-Hungary for starting the First World War. These discussions have their uses, but I would caution against getting too heavily invested in them. Explaining war is never as simple as saying "nation X started it because of event Y", and the First World War is a prime example of just how dangerous it can be to make such a bold statement.

Simply put, the opinion on the role of various factors which may have contributed to the cause of the war is almost directly entwined with the responsibility factor of historiography for the First World War. In other words, many historians of the post-war and even modern schools ask "Which nation's policy was more detrimental to European peace?" rather than "How did policy itself doom the nations of Europe into the First World War?". Whether or not this method is historiographically valid remains up for debate, as it certainly elicits a certain sense of bias within both the historians and their source material. Even as early as 1973, John Röhl claimed the following on developments in historiography linking to the Great War:

The controversy over the immediate causes of the First World War appears to be at last drawing to a close.

Perhaps Röhl may simply have been looking at the recent writings of historian Fritz Fischer, whose hallmark 1961 publication Griff nach der Weltmacht (Germany's Aims in the First World War) seemed to be a sign of things calming down in academia. Sadly this calm never lasted for very long and we now have a slew of new German historians who look beyond their German Empire and the aggressive Weltpolitik (world politics) of Kaiser Wilhelm II to the policies of other countries (France, Britain and Russia among them). Add on top of that the developments alongside Röhl's own writings with historians such as AJP Taylor and Zara Steiner who posited that it was a "war by timetable" and "collective fault" respectively. Policy does take a central line in almost every writing on the First World War, as to ignore the policy of any nation being investigated is to allow the elephant to remain in the room.

For that matter, policy alone cannot explain a fair few decisions made by those statesmen and diplomats in the summer of 1914. I will not dabble too much into those considerations and choices here, but instead link my own archived responses which should help make it even more clear just how many various theories, explanations, and factors must be taken into account when "determining" the cause of World War 1. I have also attached some further reading and watching should you, dear reader, be interested in going down this rabbit hole of historical writing and debate. I warn you well in advance however, that you (like myself and countless other historians) shall never emerge from this rabbit hole with an end-all explanation, and that your fluffy tail will be dirtied considerably in the process.