"Knowing others is intelligence; knowing yourself is true wisdom" ~Lao Tzu
If you are going to war against a country larger and more powerful than you, you will need allies. At least one, preferably more. In 1914, no one was more aware of this than France. Germany and its satellites were at least 20% larger than France and its population was half again as large. France's primary ally, really its only confirmed ally, was Russia.
Russia, however, was a constant cause of trepidation to France because it refused to confirm just how much it would thrown in with France when war began. Russia wasn't intending to be malicious with its vacillation, it had its own problems to deal with, and its sheer size and distance from Germany made the way it could wage war different than France. If Germany started a two-front war, France was immediately in the cross-hairs. It would be the first thing Germany went for. Russia was so far away that by the time they reached even the borders of Germany, France could have fallen. Russia's distance from Germany allowed it to wait a few days to mobilize. It was also large enough that it could endure a initial (relatively) small loss of territory to the Germans while calling up their reserves.
Size and distance aside, Russia also had its own problems at home without having to worry about its comparatively small ally. They had just come through a humiliating defeat by Japan in the Russo-Japanese War in 1904-05. That war, considered the "first great war of the 20th Century" was an imperial dispute between Russia and Japan over Manchuria and Korea. After that, Russia had a minor revolution in 1905.If Germany had implemented its Schlieffen plan in 1906, it probably could have wiped out both France and Russia. But by 1909, Russia was starting to pull itself together again and work out its own mobilization schedules in the event of another war. Indeed, the mistakes Russia learned from in 1904 helped it to defeat Germany in 1917. After much waffling, Russia surprised everyone by throwing in its lot with France in 1913. There is still speculation today as to why Russia seemed to have a sudden change of heart. One possibility is that Russia had an Austrian double agent that sold them the secrets of Austria's war plans. Another is that Russia finally got over its defeat by Japan and was militarily and economically to the point where it could start expanding again. Or perhaps Russia just realized that if France fell, it could not stand alone against the combined force of Germany and Austria-Hungary. Whatever the cause, the idea of having Russia coming at them from behind while they attacked France certainly gave Germany pause.
Germany was not without its own ally problems. Germany's main ally was its smaller associate to the south Austria-Hungary. Austria-Hungary vacillated on whether it would or would not help Germany in the next war. While Austria-Hungary was more interested in taking on Serbia, Germany intended to throw Austria at Poland to keep back the advancing Russian army while they dealt with France. It was not until summer of 1914 that Austria-Hungary finally agreed to engage Russia, and only if Germany guaranteed they would grant them a portion of the German forces to assist.
Russia aside, the one thing Germany did not consider was Britain getting involved. France couldn't really count on Britain either, because Britain had strongly implied that they did not want to get involved with anyone else's problems. Britain had an advantage that the rest of Europe did not have, isolation. They were separated from everyone else by water, while the rest of the countries were separated by a river or mountain range at best, at worst, just a line on a map. While the rest of Europe had to worry if a conflict elsewhere would spill over into their land, Britain could stay out or choose to get involved, and with whom. Britannia also had the largest and most powerful navy in the world, although Germany was fast catching up with them. Indeed, if conflict did start, that is how Britain at first planned to engage Germany, by putting its impressive British Navy against the German High Seas Fleet. Over time, however, the British government and military came to realize that if Germany was the aggressor, the decisive point of battle would lie in France, and it was there they would have to go. They came to an agreement in 1911, and France added Britain into their battle plans, while Britain went to work determining the fastest way to send soldiers over the Channel. France and Britain had never really gotten along, in the past they went to war against each other as often as not, but this time they would operate on a "the enemy of my enemy is my friend" policy. Stop the enemy from invading you before he invades us both.
Everyone had been making plans and stockpiling weapons and men for years. Planning moves and counter moves, trying to anticipate the other side's reaction to their advance. The board was set. All that was needed now was the opening move.