World War I was caused by tensions that had gripped Europe for years. Prior to 1914 there had been many changes effected in European geography.  Beginning in the 1860s Prussia, in a series of short wars, began to conquer the separate Germanic states.  Prussia then defeated Denmark; furthermore, with its Germanic tribes and Denmark under its rule, Prussia, was able to defeat France, thus chaninging the political geography of Europe. 

Following the Franco-Prussian War, the Germanic states unified their twenty-six independent states into the German Empire, declaring Prussian King Wilhelm I the Kaiser and Bismarck the chancellor [like a prime minister].  Wishing to penalize France for its recent defeat, Germany claimed Alsace-Lorraine for its own and forced the French to pay reparations.  This act led to implacable hatred between France and Germany.

Hoping to avoid having to fight both France and Russia, Germany sought to form an alliance with Russia.  The emperors of Germany, Austria-Hungary, and Russia pledged friendship to one another in the Reinsurance Treaty of 1887; however, this did not last after the Kaiser died and Bismarck, who was architect of the peace, left office.  Relations soon broke down, and Russia formed an alliance with Germany's enemy, France.

When an Austro-Hungarian Empire was formed, it was destined to failure as the Hungarian Empire was composed of ethic groups left over from the old Ottoman Empire that had extended across Europe.  Consequently, the ethnic divisions among Serbs, Slavs, Croatians, Romanians and Austrians led to the break-up of this empire.  Here conflicts developed from changes made in geography that had moved peoples from their original homelands.

Then, too, across the English Channel, lay the country of Great Britain, a puissant country with the greatest empire in the world.  Certainly, Britain did not want Russia or Germany or France to expand their empires, so it kept watch over them and protected its own geography by allying itself with Russia and France.

I would say that human geography played the largest role in the outbreak of WWI.  Specifically, WWI was caused in large part by the fact that some ethnic groups were being ruled by people of other ethnic groups.

The war started when Slavic nationalists assassinated the Archduke of Austria-Hungary.  The ruling family of that empire was ethnically German, but many of its subjects, especially in the Balkans, were Slavs.

When Austria-Hungary tried to punish Serbia (a Slavic country) for its role in the killing, Russia (another Slavic country) came to Serbia's aid.  Then Germany came to Austria-Hungary's aid for ethnic reasons as well.

How did geography affect World War I plans of the participants?

The geography of the border areas had every effect on war plans.  The Germans, since the War of 1870, were certain that another war with France was inevitable, with much larger armies. French and German armies at the beginning of the war numbered over one million troops.  The German plan, masterminded by Count Alfred von Schlieffen, involved a mass advance across the entire frontier of France from the Swiss border to the English Channel, with the right wing heavily weighted.  While the German left and center would hold the French in place, the German right would swing through Belgium and descend upon the French rear, crushing their armies and seizing Paris in six weeks.  The plan involved violating the neutrality of Belgium; there was no other way of getting the troops into position above and behind the French army.  They realized this would bring Britian into the war, but the British army was very small in comparison, and the General Staff believed the war would be over too soon for the Brits to be a factor.  They also believed that the Belgians would offer only token resistance.

The second part of the German plan involved Russia, which also hinged on geography.  The only possible approaches to East Prussia was for the Russian force to split in two, to pas north and south of the Masurian Lakes.  The Germans planned to retreat, sacrifing eastern Prussia, and hold the Russians until the decision was won in the West.

The French plan was the infamous Plan 17, possibly the worst war plan ever made by a nation.  They intended to attack directly into Germany regardless of the German right wing.  This was foolish for three reasons: 1, the Germans would descend en masse on their rear; 2, on the French left (German center) they would be attacking into the Ardennes, an area with few roads and difficult terrain; and 3, on the French right they would be attacking into the Vosges, a mountain range no one (until the American 7th Army in WW II) had ever seized from an opposing force in all of history.

The British plan was to take their very tiny army and attach it to the French left wing, where they would be outnumbered massively as well as outflanked by the Germans.

The Russian plan was simply to throw themselves on the Germans in East Prussia, the First and Second Armies passing on both sides on the Masurian Lakes with no hope of coordinating their movements.  Against the Austro-Hungarians, the Russian plans were better, although they were nothing more than to counter the obvious movements planned by the Austrians.  Through the use of a spy (who had been head of the Austrian intelligence service) they knew the full Austrian plans, which were simply to attack along the most obvious geographic lines.

The Italians didn't really have much in the way of plans.  They were a member of an alliance with the Germans and Austrians, but did not take part in the war initially and then entered on the side of France and Britain.

The Bulgarians, Romanians, etc. had no plans for large scale warfare involving a general European war, just the usual Balkan plans to seize whatever they can.