The Austro-Hungarian empire declared war on Serbia after the archduke was assassinated. Russia mobilized its armies to help Serbia, and consequently Germany declared war on Russia, too. France, being in an alliance with Russia, automatically entered into a war conflict, and finally Britain entered into a war, too, because of the German conquest of Belgium.
EXPLANATION
The war developed on two fronts: the Eastern Front and the Western Front, which highlighted the trenches wars. On the western front, the French mobilized their troops on the French-German border. The Germans, however, came up with a plan called the Schlieffen Plan, which would be used to defeat the French army. In the first Battle of Marne, which took place between September 6 and 13, 1914, the French succeeded in stopping the advance of the Germans, which led to the Schlieffen Plan becoming a war of positions .
On the Eastern Front, the German war strategy also worked against Russia. Russia did not have a well-trained army as there were large numbers of soldiers who were only armed peasants. The Russians suffered multiple defeats in different wars. The Japanese empire also entered into war with the Allies on the side of conflicts with China.