The short answer is that Germany was itching for a fight with France. The assassination was nothing. If Germany hadn’t wanted a war with France there would have been a lot of words, but nothing. But when it looked like war was possible, Germany begins mobilizing, and staff officers all over Germany are breaking open the seals on their part of the Schlieffen plan. And with Germany mobilizing France mobilizes.
These mobilizations are the key factor in the war. Everyone can see that the sheer sizes of national armies in 1914 is much larger by far than anything in the past, and that due to modern transportation and communication armies can be assembled in weeks. And if you have a massively mobilized army sitting around, and the other guy doesn’t, you can just walk in and brush past the standing army defenses. But of course this massively mobilized army is horrendously expensive, since you are taking the bulk of the productive work force of the country and forming an army out of it.
So you absolutely cannot have your mobilized army sitting there for nothing, the country will be ruined. Either they fight, or they go home. But if they go home, what happens when the other guy mobilizes, and now you’re reversing your mobilization? You can’t, the mobilization plans can’t work ad hoc, you need extensive planning to get troops and horses and gear to the right place at the right time. So the logic of the situation says that as soon as you possibly can you must attack. And the thinking of the time was that the attackers had every advantage over the defenders. Fortifications were all well and good, but could be crushed or bypassed at will by concentrations of attackers.
And looking at how the Germans brushed through Belgium they were perfectly right. The Belgian forts were reduced within a few days. Of course the Germans are going through Belgium so they don’t have to attack the fortifications in France, but even in southern France they are making progress. But of course the main push is through Belgium and on to Paris, everything else is a delaying action with all possible strength put on this offensive.
And the thing is, the Schlieffen plan very nearly worked, the French were on the very ragged edge of collapse when the German offensive faltered. It really could have been the short sharp victorious war and back home for Christmas that everyone expected, it would have been that if the Germans had got to Paris. Compare and contrast WWI, where the offensive halted just outside Paris, and WWII, where the Germans take Paris and France falls and everyone is home for Christmas just in time to be sent to Russia.
Everyone makes a big deal of the trench warfare and how in WWI technology favored the defense. Except at the beginning of the war this wasn’t so. You need massive defense in depth to neutralize the attackers, if they can break through your lines to your rear they can just march on Paris or cut your supply lines. You cannot just sit there and let them attack you. In the early war the massively prepared fortifications don’t exist yet–or rather, there were fortifications but they were bypassed and the French troops manning the border fortifications are useless because they aren’t being attacked, the vast bulk of the German army is pouring through Belgium and into your rear. To be effective you have to abandon your positions and head north to meet the German offensive, except when you do that you don’t have massive trenches, machine gun nests, barbed wire and artillery behind you, you’re marching for days until you flop down exhausted in the mud and have a few hours to dig a hole for yourself.
Your only hope is that the Germans attacking you have been marching for weeks and are utterly exhausted themselves. The army that crushed Belgium like a grape is out of food, ammo, sleep, the men can barely walk. But the same with the French defenders. It came down to a coin flip, and the German advance halted, unable to go on. But if they had managed to scrape up a tiny bit more strength, and Paris had fallen, then all this modern talk about the futility of bravery in the face of machine guns would have never happened. Instead it would all be about elan, and how with enough elan and spirit you could accomplish anything.
And the history of Germany’s quick victories in early WWII show this is quite right. The technology 20 years later wasn’t that radically different, and machine guns still didn’t care how brave you were. Except the Germans managed to smash through country after country, destroying their ability to mount a defense, in just a few weeks. The only time this didn’t work was against Russia.