Germany entered the First World War in the unenviable position of having to fight on two fronts. Their battle plan, a modified version of the Schlieffen plan, called for a defensive war on the Eastern front with a vigorous attack in the West bringing the hostilities there to a rapid and decisive end.
To carry out this plan seven armies stood ready, two on the left flank which would fight a holding action in the Alsace-Lorraine area while the five on the right flank would march through neutral Belgium, circumventing the vaunted French defensive lines and outflanking the allied troops in a massive right hook. It was expected, hoped, that the French would attack in the Alsace-Lorraine area. The weaker German defenders would give way and entice an over enthusiastic French high command to leave the safety of their fortresses and advance into German territory. In the meantime the right hook of the Schlieffen plan would swing around into French territory and come up behind the advancing French troops.
The first fighting took place in Alsace-Lorraine. The French troops suffered terrible losses in attacks that were bravely carried out, but they were using tactics that had no place on a modern battlefield. The German commanders in the area were loath to let the French advance and not wanting to miss out on the glory, chased the French back into their defensive lines, creating a front line that would remain almost unchanged until the end of the war.
Then the offensive for Northern France and Belgium broke loose. The taking of Lüttich (Liege) was like popping a cork that allowed the German troops to flood into Belgium and a series of short but furious battles followed giving the Germans numerous tactical victories but failing to rout the French who managed to retreat over the Marne in a reasonably orderly fashion and regroup. At this point the German 1st and 2nd Armies were wheeling in to the northeast of Paris hoping to give the coup de grace to the French armies. During the march a gap developed between the 1st and 2nd Armies which von Kluck tried to rectify by shifting troops to fill it. By doing this he weakened his right flank which did not go unobserved by the French.
On the 6. September the French attacked the 1st Army causing von Kluck to rush three corps in to protect his right flank. This in turn helped create a new gap between the 1st and 2nd Armies and the British and French troops attacking in the Chateau Thierry area rushed into it dividing the Armies. It was the beginning of the counter attack which the French would call "The wonder on the Marne" and which pushed the German Armies back behind the Aisne and to the north of Reims. On reaching this point the Germans dug in.
A static front had developed from Soissons to the Swiss border and it rapidly became evident to both sides that a flanking movement in the West was needed to turn the tables on the enemy. This was not to happen. Although both sides threw all possible reserves into the fray, a series of leapfrog battles that became known as "The race to the sea" developed with milestones in the race being battles at Noyon, on the Somme, by Arras and on the Yser. Neither side was able to get around the other and when the Belgians flooded the coastal area the last hope for a flanking movement died and from that moment on the Western Front settled down for the next 4 years, 700 Kilometres of trenches with a wedge into enemy territory at St Mihiel (the Fort Camp des Romains).
At the beginning of the war East Prussia had a single German army to defend it. The soon to be famous Duo of General Paul von Hindenburg and his chief of staff Erich Ludendorff ignored the Russian 1st Army that was threatening her flank and rear and threw their divisions into the path of the advancing Russian 2nd Army, destroying them at Tannenberg. After Tannenberg they turned their attention to the Russian 1st Army and although not destroyed the Russians suffered serious losses.
In the meantime the Austrian army was suffering defeats in Galicien while supported by the Landwehrkorps Woyrsch. Hindenberg rushed south with the majority of his forces and helped push the Russians back to the Weichsel river and to Warsaw. In a well coordinated retreat he then avoided a Russian flanking movement.
General Von Mackensen led a second campaign in Poland and won victory after victory until reaching Lodz. Here he was almost defeated but managed to keep strategic control and the campaign ended on the Bzura, Rawka and Pilica Rivers, the Russians beaten for the moment.