If you would like a summary of the leadup to the war check out this post!

• Why was WWI considered "inevitable"?

For your question in particular, Germany declares war on France because Germany wanted war with France. We need to drop all the pretense in these discussions and get to the gritty details: By Winter 1912 Germany had decided upon war with France and Russia and was looking for a reason. While this 'war council' did not reach anything resembling a legal, binding decision it did create clear statements of intent from the big decision makers high in the German government. Immediately following this meeting a propaganda campaign was ordered to garner public support for an invasion of Russia, the navy was ordered to prepare imminent naval war with Britain while the navy was given an expansion (Tipitz asked for a "postponement of the great fight for one and a half years" to prepare the navy; coincidentally the war began 1.5 years later). Food would begin to gathered for war as mobilization studies and committees were formed and officers were prepared for imminent war.

Ultimately this "yet another Balkan conflict" had no reason not to be yet another little Balkan conflict. However Austria-Hungary was determined not to let the last moment slip by to 'settle accounts' with the Serbs.^1 They wanted their 'final and fundamental reckoning' with Serbia.^2 The leadership in Vienna saw their empire in rapid decline and would rather burn out brightly in 'apocalyptic war' rather than fade away into obscurity and thus they got their excuse when the Archduke was assassinated. They were getting their war one way or another and Berlin saw her opportunity as well. The Russians were already into their remilitarization and reindustrialization program which was slated to finish in 1917 and the Germans wanted to hit them hard before they got eclipsed.

Serbia was their excuse; despite the Russians only mobilizing in provinces bordering Austria-Hungary and the Tsar's insistence that the conflict would not bleed into Germany, Berlin sent a letter demanding all border forts be destroyed, border provinces be occupied as a sign of good faith of neutrality, the Russian military demobilize, and that German officers oversee this whole thing. They would send a similar letter to France demanding the Verdun and Toul fortresses be occupied by German troops as a sign of 'good faith' while they were also deconstructed. Obviously these were designed to be refused so that when they were refused Berlin could paint the Russians and French as aggressors. I don't got the book with me right now so I'm punching myself but reading the accounts in Herwig's Germany and Austria-Hungary, 1914-1918 of German government officials literally shaking hands and cheering that they get to paint the Russians as the aggressors is chilling. It's almost comic book level of blatant obscenity.

So now that war was certainly coming even then France's government refused mobilization. In fact the opposite -- they were petrified as being portrayed as the instigators of this war. They pulled their forces back 10km from the German and Belgian borders even to not have any accidental border skirmishes. Germany would goose step around total mobilization by ordering "imminent danger of war" which was, functionally, mobilization but limited. This would be the final straw for the anti-chauvinist French government. They would both order mobilization within literal moments of each other -- at 4 P.M. Parisian time August 1st, 1914. France when they got no reply from the German ambassador inquiring about this pseudo-mobilization and Germany when their 12 hour ultimatum to Russia expired.

---

^1 Conrad's audience with Franz Joseph, 5 July 1914. OStA-KA, Operations Buro, General Stab 91.

^2 Tschirschky to Bethmann Hollweg, 30 June 1914, cited in Imanual Geiss, ed., Juli 1914: Die Europaische Krise und der Ausbruch des Ersten Weltkriegs (Munich, Deustcher Taschenbuch Verlag, 1965), pp. 39-40