Oh boy is this a basket case of a question and I'm basically going to cover as much of this topic as I reasonably can.

Let's start in 1870 when Prussia and various other German states declared war on the Kingdom of France under Napoleon III. Prussia had just four years prior seized many territories in the 1866 Austro-Prussian War and formed a confederation of North German states -- the North German Federation. The war on face value was fought over Prince Leopold, a Prince of a German minor state, who was to succeed the Spanish throne. The French feared total encirclement by part of Germany and a German ruled Spain. Although Prince Leopold would withdrawal his candidacy as successor, underneath Bismark and Willhelm I wanted war desperately to unite the German states -- especially the Southern Germanic states of Bavaria and Württemberg -- under a common banner of "Germany" and further knocking the French whom they saw as an issue of European stability down a few pegs.

So, Prussia & Friends went to war on the guise of Spanish succession and a few political "blunders" (wink wink) and they besmirched the French. The French would in the first year of the war have essentially their entire army encircled and captured at once at Sedan. This would be very important later on in the First World War as the children of the men who performed this amazing military feat would desperately try to live up to their fathers legacy. Nonetheless Napoleon III was captured as well and France declared herself a Republic and continued the fight fruitlessly until January '71 when Paris fell. Germany was declared an independent state with Willhelm I of Prussia as its king and also seized German majority areas on Alsace-Lorraine from France.

What followed was the most complex topic in history basically, which is Bismarkian politics. This also requires going a bit back in time. In 1815 the Treaty of Vienna was a concrete end to the Napoleonic Wars and created a balance of power in Europe by organizing which states would have which territory that warded off what was called a "General War" -- as in a war between all the European states -- for a damn long time. Until 1914 actually. Germany upset this balance by winning that war and declaring themselves as an independent state. It twisted everything on its head.

Germany went overnight from a conglomerate of tiny states and kingdoms into a world power overnight. I mean that very literally. They would form as a country in 1871 and by 1880 (arguably 1875 if I recall, but certainly 1880) was the world industrial leader in both output and capacity. This lead would persist until the United States took over in the 1900-10 decade but only by a slim margin. They would also overnight have the largest land army in Europe at the time and arguably the most professionally organized in the world. I want you to combine all of this with the common phrase that Prussia was not a country with an army, but an army with a country. To say Prussian Militarism dominated the German psyche would be the understatement of the century.

Bismarkian politics was Otto von Bismark's attempts to avoid reprisal against Germany. He recognized the fragile situation that he and William I had created and devised a system to avoid a general European war. That is, the 3/5th's rule. Germany must remain part of a 3 part alliance of European allies out of 5 total -- Britain, France, Germany, Austria-Hungary, and Russia. Germany would lock up Austria-Hungary, Italy as a secondary power to further bolster its alliance, and meet that 3/5th's rule by locking in Russia. Russia and Germany had less of an alliance but more of a mutual non-aggression pact and at best a defensive treaty much like Germany held with Italy -- which Italy easily opted out of once the cards were laid on the table.

This pissed off France to say the least. Germany viewed France as its greatest enemy and its greatest potential threat ideologically and did everything in her power to keep France in check. This would all go crashing down in the late 1890's after Willhelm II ascended the throne. He would sack Otto von Bismark, wanting to make his own mark on the world. He would, in the process, completely alienate Russia and create the situation Bismark feared the most -- German encirclement. Within days of Germany pushing Russia away intentionally France would swoop in and create a mutual defense pact with Russia. However, Great Britain was still out of the equation. It was, essentially, Austria-Hungary and Germany versus France and Russia -- Britain had what can be basically taken as a handshake agreement but nothing set in stone.

As much as I'm itching to go into the struggle between Germany and Britain in the 1900's and 1910's this is focused on France so this is basically where our story cuts off. The summary is that everything Germany tried to do to create a divide between Russia, France, and Britain ultimately created an even tighter bond and would create a situation where Britain went from being isolationist from Continental issues into directly looking for a reason to support France and began drawing up plans to assist in a land war because of Germany's actions.

Germany realized in about 1904/05 that it needed a military solution to its encirclement that was the Schlieffen Plan. Schlieffen the Younger wanted to live up to the legacy of his father, as I mentioned earlier, and drew up a convoluted plan to attempt to complete it. The German army would mobilize fully and then send the entirety of its forces except for a few token forces to sweep through Belgium -- a 7:1 ratio between the Belgian forces and the forces holding the Franco-German Border even. The German forces on the border would tactically retreat and the forces in the North would rapidly seize Paris and then sweep down and encircle the French near the border -- the swinging door strategy as it's called. Then once France would be knocked out quickly -- in under 900 hours by the plans designation -- Germany would then swing her armies all East to meet the Russian juggernaut that would just be finished mobilizing.

Well, von Moltke realized how fantastical this plan was and reduced it down a bit -- some would say tainted. The principle remained the same however -- knock out France first and this meant going through Belgium for the quickest possible way. This would be the diplomatic justification for Britain joining the war and needless to say, the Germans got stuffed and the Russians were pushing into East Prussia much earlier than expected. The plan fell apart and from then that leads to the war of attrition we know and love today.

This is a very rough outline of events though. If you want me to go more in depth in any area please ask!

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Notes:

Strachan, Hew, "The First World War: Volume I: To Arms"

Mosse, W.E., "European Powers and the German Question, 1848-1871"

Carr, William, "The Wars of German Unification"