Germany attempted to follow an aggressive military strategy known as "The Schlieffen Plan" where it started the war by attacking in two fronts. German army wanted to invade France in the west and capture the capital city, Paris through Belgium and attack Russia in the east.
The strategy was to deploy most of the soldiers in the army in separate troops to the west front and invade France. Once France came under control the west front army would be sent to east to help the east front German army in attacking Russia.
French army initially underestimated the strength and strategy of the Germans. Germany deployed reserve troops along with their regular troops which was totally unexpected.
Germany started an offensive attack to penetrate into Belgium on August 4, 1914 and entered France. German army had initial clashes with France and Belgium forces in the "Battle of Frontiers".
Germany was successful till the Battle of Frontiers. In "First Battle of the Marne" French army halted the advancing German army with the help of British forces. Belgians too fought back and damaged the rail system which caused an unexpected delay in Germany's movement.
With the enemies attacking offensively, the German army was supposed to fall back. But the crown prince of Bavaria ordered the troops to counter attack.
If Germany played defensive on the western front, it would have tasted the victory. But because of the communication problems and questionable commands for personal glory cost Germany the chance of obtaining early victory.
Russian forces invaded the German-held regions of East Prussia and Poland. But were stopped by the German and Austrian Forces in the "Battle of Tannenberg (17 August - 2 September)".
Germany defeated Russia in Battle of Tannenberg and safeguarded East Prussia with the help of additional German troops from western front.
Germans switched the bulk of their forces from Prussia to Poland to counterattack Russians with Austrian troops.
Russia's attempt for a massive operation became a failure and by that time four new German army troops had arrived from the western front.
Disorganization in the army and depletion of their munition supplies compelled Russians to fall back.
German army too suffered from sabotaging their rail networks. Soldiers suffered from lack of food as the British naval blocked the food imports to Germany.
Combined with the fierce attack of the allied forces in France, Russia's quick mobilization with huge war machinery ensured a longer conflict instead of quick victory than was expected from "The Schlieffen Plan".
Meanwhile a part of Austrian army invaded Serbia in the mid August. After a series of attacks, Austrians captured and evacuated Serbs from Belgrade on 30 November, 1914. But by 15 December, a Serbian counter attack had retaken Belgrade and forced Austrians to retreat.
These failures turned out to be a heavy blow for the Central powers.
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