Due to the global expansion of the power system, fighting took place on almost all continents and most of the great seas, as well as in the air. The focus of the fighting on land was Europe with two main fronts (western and eastern front) and two secondary fronts (southeast with changing locations: Serbia, Romania, Thessaloniki front; southern front: Italy, from 1915). In addition, there were three fronts of the Ottoman Empire as an extension of the southeast front: against Russia in the Caucasus and in Armenia, against Great Britain in Iraq and on the Suez Front (or from the end of 1917 in Palestine), temporarily also in the battle for the straits (1915).
Balance of power at the beginning of the war: Because in Germany – unlike in France – almost half of the conscripts were not called up for military service, France had a field army of 1.86 million men (with 36.6 million residents) against Germany with 2.29 million men (with 67 million residents) and Austria-Hungary with 1.42 million men (with 51 million residents). Since Russia and Serbia together raised around 3.6 million men, the Central Powers, with around 3.7 million men, faced around 5.8 million land troops from the Allies (including Great Britain and Belgium).
The war year 1914 – from movement – to positional war on the western and eastern front: According to the Schlieffen plan modified by H. von Moltke the German deployment took place with a focus on France (7 armies in the west). In order to bypass the fortified belt of France and defeat its army in a large encirclement operation, the crossing of neutral Belgium with a strong right wing was planned. After the occupation of Luxembourg (August 2nd) and Liège (August 7th) began on August 18th. the advance of all German armies (August 20, capture of Brussels). The border battles at the end of August brought no decision; the German troops crossed the Marne east of Paris at the beginning of September. The successful counter-offensive of the Entente (6–9 September) between Paris and Verdun forced the German armies to retreat behind the Aisne. The Battle of the Marne was the first turning point in the course of the war; With it the German strategy to avoid a war on two fronts failed; there was a leadership crisis in the Supreme Army Command (OHL) of Germany (14.9. Chief of the General Staff H. von Moltke replaced by E. von Falkenhayn ).
Mutual overflight attempts in autumn extended the front to the North Sea (“Race to the Sea”). In the following two battles in Flanders (October / November) with a focus on Ypres, all German attempts to penetrate the French Channel coast failed. After that, the western front froze in trench warfare and trench warfare.
According to allpubliclibraries.com, the German 8th Army joined the Russian Nyemen Army, which had invaded East Prussia, on August 20. at Gumbinnen against. At the news of the approach of the Narew Army, the German General M. von Prittwitz broke off the battle and ordered the retreat behind the Vistula. He was replaced by General P. von Hindenburg (Chief of Staff General E. Ludendorff ), who decided to attack the Narew Army.
After their destruction near Tannenberg (August 26–31), Hindenburg turned against the Nyemen army and defeated them at the Masurian Lakes (September 6–14).
The Austro-Hungarian army advanced on Lublin and Lemberg, defeated Russian troops at Kraśnik (August 25th) and Komarów (September 1st), but had to retreat into the Carpathian Mountains and behind the Wisłoka in September from the overwhelming Russian forces. The German 9th Army, formed from parts of the 8th, was deployed under Hindenburg north of Krakow to support him. In October it advanced as far as Ivangorod and south of Warsaw; Threatened by encirclement, she withdrew to Upper Silesia. As a result, the Austro-Hungarian armies that had meanwhile advanced to the San and to Ivangorod had to be taken back to the Carpathian Mountains and Krakow. On November 1st became Hindenburg Commander in Chief East. He left the protection of Upper Silesia to the allied army and weak German forces and put the 9th Army under General A. von Mackensen ready to attack at Thorn. began and led to the encirclement of several Russian corps around Lodz (battle from November 17th to 25th). With the advance of strong Russian forces from the northeast, the German left circumferential wing was enclosed. He made his breakthrough on Brzeziny (November 24th). In December the Russians were pushed back behind the Bzura and upper Pilica and near Krakow. After that, the eastern front also switched to trench warfare.
The campaign against Serbia began on August 12th. with an unsuccessful Austro-Hungarian advance across the Drina. A new attack in September resulted after heavy fighting on December 2nd. for the capture of Belgrade; as a result of a Serb counter-attack, it ended with a retreat behind the Drina and Save.