Greetings! The idea of Germany joining the First World War is certainly something which many people might dismiss as an "automatic" or a "given" when first glancing at the facts, but the reality could not be further from that assumption. In this response we'll delve a bit deeper into the sequence of events and factors which prompted Germany to enter what would eventually become the First World War. Let's begin.

As a sidenote to OP, the reason you were unable to see the other comments on your first post was not because of a bug, but rather due to the active moderation which takes place on AskHistorians. The moderators here (human and bot) do an excellent job of removing comments which are insufficient in detail, lacking depth, or simply do not address the question at all. As you'll see in this thread, u/DanKensington has actively removed a comment which did not comply with AskHistorians' rules.

A Shooting in Sarajevo

"Who authorised him to do so? This is utterly stupid! It is none of his business, since it is entirely Austria's affair [to determine] what she intends to do. Later on, if things went wrong, it would be said: Germany was not willing? Will Tschirschky [German Ambassador to Vienna] by so kind as to stop this nonsense! It was high time a clean sweep was made of the Serbs."

- Kaiser (Emperor) Wilhelm II's marginal comments on a dispatch from Ambassador Tschirschky regarding his de-escalatory remarks to the Austro-Hungarians, 30th June 1914.

The above comment is one which stands out in the weeks prior to Germany's war beginning in 1914. When read for the first time, one might takeaway that the Kaiser was an aggressor in the whole affair between Austria-Hungary and Serbia, and that he actively desired war as an outcome of the assassination of Archduke Franz Ferdinand. Taken on its own the source certainly supports such an assumption, but in the wider context it is an anomaly.

We must remind ourselves here that the German Empire was part of the Central Powers Alliance with Austria-Hungary and Italy (though the latter would abandon the alliance in 1915). Under the conditions of this mutually-binding defense pact, the Germans would come to the aid of the Austro-Hungarians if they were attacked by a third party, and vice-versa. When it came to being the party that attacked another country however, the alliance did not contain any obligations for the powers to aid each other militarily. It was this consideration that pestered the decision-makers in Vienna and (to a lesser extent) Budapest in the days immediately following the shooting, as historian Christopher Clark remarks:

Finally [for the decision-makers], and most importantly, there was the question of Germany. Would Berlin support a policy of confrontation with Belgrade? German backing for Austro-Hungarian Balkan policy had been patchy of late.

The Germans were sending mixed messages as June turned into July. We already mentioned Baron Tschirschky's urging of caution in Vienna, but a noted journalist by the name of Viktor Naumann reported to the Austrian cabinet that Germany would "look kindly" on an Austro-Hungarian strike against Serbia (Naumann was known to have links to Wilhelm von Stumm, head of the political department of the German Foreign Office). It was therefore decided that before Austria-Hungary took any further steps against/regarding Belgrade, that Germany had to be properly consulted and her support won over. The documents were prepared, and Count Hoyos was dispatched by night train to Berlin on July 4th. It was here that several critical decisions would be made that moved the German Empire one step closer to the brewing storm.

The Blank Cheque and the Russian Bear

The Chancellor [Bethmann-Hollweg] speaks of weighty decisions. The murder of Franz Ferdinand. Official Serbia involved. Austria wants to pull itself together. Letter from Franz Joseph with enquiry regarding the readiness of the alliance to act.

It's our old dilemma with every Austrian action in the Balkans. If we encourage them, they will say we pushed them into it. If we counsel against it, they will say we left them in the lurch. Then they will approach the western powers, whose arms are open, and we lose our last reasonable ally.

- Kurt Riezler, diplomat and confidant to German Chancellor Theobald von Bethmann-Hollweg, recounting a conversation with the Chancellor on the night of July 6th.

Hoyos arrived in Berlin the following morning, and after several meetings with the Kaiser, Bethmann-Hollweg, and other key members of the Imperial German Government, the infamous "Blank Cheque" was 'issued' to the Austro-Hungarians. Under this reassurance, the German government had assented to support Austria-Hungary in whatever actions it took against Serbia. Note that this did not constitute German pressure for Austria-Hungary to go to war against Serbia; though that radical course of action was certainly a possibility that all decision-makers involved were aware of.

In fact, up until the very last days of peace, the German government believed the war could be localised. That is, a "Third Balkan War" of sorts, involving Austria-Hungary, Serbia, and with limited Russian intervention. The Kaiser himself was confident in this possibility, noting to Admiral von Capelle before leaving Berlin on July 6th that he:

"did not believe there would be any further military complications."

Why the Germans underestimated the Russian desire to get involved in the conflict has been studied by many historians, and the resulting conclusions they have come up with might be best summarized by the "Preventive War" argument, as Clark outlines below:

"The argument in favour of launching a preventive war was consisted of two distinct and separable elements. The first was the observation that Germany's chances of military success in a European war were diminishing fast; the second was the inference that Germany should address this problem by itself seeking a war before it was too late...the evidence that suggested diminishing chances of success also implied that the risk of a Russian intervention was minimal. If the Russians' chances of success in a war with Germany really were going to be much better in three years' time than they were in 1914, why would St. Petersburg risk launching a continental conflict now, when it was only half-prepared?"

In other words: Germany's decision-makers saw little reason why their Russian counterparts would actively escalate the situation as it stood, since that would ultimately cause the French and British governments to involve themselves in the situation, thus raising tensions to a point where continental war (as opposed to a localised one) would be a serious consideration.

For the sake of brevity, I have grossly-oversimplified the Russian decision-making process, but the key thing to note with St. Petersburg's own course of action during this time was one of escalation (albeit uncertain and at times conflicting). For the final part of this response, we shall explore why Germany's decision to go to war involved a two-front conflict with France and Russia, as opposed to the Serbia-only war which OP has suggested.

Part 1 of 2