The first weeks of WWI saw a confusing array of clashes as Germany invaded Belgium, France invaded Germany, Russia invaded both Germany and Austria-Hungary, and Austria-Hungary invaded Serbia. The most successful of these attackers was Germany, which overran Belgium, defeating the first arriving British forces, then chased the French and British armies almost to Paris before finally being stopped at the Marne.
Following Germany’s declaration of war, France invaded German-ruled Alsace-Lorraine but was beaten back by 24 August. In the meantime, the Germans pushed through Belgium, defeating the newly arrived British Expeditionary Force at Mons on 23 August.in wikipedia
Several clashes between Austro-Hungarian and Serbian forces led to battle around Cer Mountain, northwest Serbia. After four days fighting, Austro-Hungarian morale collapsed, with thousands fleeing for the border and many drowning in the Drina River in the panic. The Serbians followed up by recapturing the town of Šabac, bringing an end to the first Austro-Hungarian invasion of Serbia.in wikipedia
Austro-Hungarian forces invaded Russian Poland, advancing on Lublin and Cholm, but were routed by Russian forces at Gnila Lopa on 29-30 August. The Russian counterattack broke through the Austro-Hungarian front, capturing Lemberg in Austrian Galicia.in wikipedia
After the Germans broke through at Mons, Belgium, the defending British Expeditionary Force and the French Fifth Army were forced into retreat. Despite a counter-offensive attempt at Guise, the Allies were unable to halt the German advance until they reached the Marne, bringing the front to within 60 km of Paris.in wikipedia
On 17 August 1914, the Russian First and Second Armies crossed the border into East Prussia, Germany. On 26 August, the German Eighth Army under Field Marshal Paul von Hindenburg and Major General Erich Ludendorff intercepted the Russian Second Army near Allenstein, making use of railways to quickly consolidate their forces. The result was the rout of the Russians, who lost 78,000 killed and wounded and 92,000 prisoners to German losses of about 15,000.in wikipedia