Germany has a plan to defeat the central powers. It is called the Schlieffen plan. Germany knows Russia will take a while to get ready for war, so first Germany will rush its troops into France while Russia's getting ready, defeat France, then move all its troops to Russia and defeat Russia, then the war is over.
One tiny little problem though. France has tons of forts on its border with Germany. Germnay decides to go around and violate Belguim's neutrality. Germany is hoping Belguim will just shut up and let thousands of troops from Germany go through it's border, but they don't and fight back. They're pretty good, too, so they slow the Germans down.
England steps in and is kind of pissed about Germany just invading neurtal countries, so they join the allied powers with France and Russia.
Germany goes into France after a successful campaign in Belguim (and a couple of atrocities along the way). They quickly tear though France. One french general orders his forces to stop retreating. In the resulting battle, a gap opened up in the German lines. When a gap opens up, the enemy can use it to flank you from the side and back, so the Germans retreat to the Aisne River.
Both sides then dug themselves into defensive trenches, running all the way from the coast to Switzerland. The Western Front was turned into a place worse than hell in less than a day.
