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.
To tell the truth the geography probably didnt matter. 1.Because of the trenches built as well as the bio ware used during the WW1 especially the blimps (called "Zepplen"?) and bomb England.
2.Most battlefields were flat and snipers had easy targets.
3.Germans had the flanks anyways.
4.The allies made the worst plan ever.
Pretty much the only possible answer is the U.S. intervention because the Germans kept sinking there civilian ships harming countless innocent people.