Many countries participated in world war one, there was Germany, Austria-Hungary and the Ottoman Empire (the so-called Central Powers) against Great Britain, France, Russia, Italy and Japan (the Allied Powers), the allies were then joined by the USA in 1917.
June 1914, Archduke Franz Ferdinand of Austria was assassinated by a Serbian nationalist in Sarajevo, Bosnia this led to the escalation of threats and mobilisation orders followed the incident, leading by mid-August to the outbreak of World War I.
In the Western Front, Germany began fighting World War I on two fronts, invading France through neutral Belgium in the west and confronting mighty Russia in the east. In the First Battle of the Marne, fought from September 6-9, 1914, French and British forces confronted the invading German army, which had by then penetrated deep into northeastern France, within 30 miles of Paris.
At the Eastern Front of WW1 Russian forces invaded East Prussia and German Poland, but were stopped short by German and Austrian forces at the Battle of Tannenberg in late August 1914.
Over the next two years, the Russian army mounted several offensives on the Eastern Front but were unable to break through German lines. Defeat on the battlefield fed the growing discontent among the bulk of Russia’s population, especially the poverty-stricken workers and peasants, and its hostility towards the imperial regime. This discontent culminated in the Russian Revolution of 1917, spearheaded by Vladimir Lenin and the Bolsheviks.
The four years of the Great War–as it was then known–saw unprecedented levels of carnage and destruction, thanks to gruelling trench warfare and the introduction of modern weaponry such as machine guns, tanks and chemical weapons. By the time World War I ended in the defeat of the Central Powers in November 1918, more than 9 million soldiers had been killed and 21 million more wounded.