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World War I began on ( August, July, September, June ) 28, 1914, when ( Austria-Hungary, Germany, Russia ) declared war on Serbia. This seemingly small conflict between two countries spread rapidly: soon, Germany, ( Russia, China, Japan, Italy ), Great Britain, and France were all drawn into the war, largely because they were involved in treaties that obligated them to defend certain other nations. Western and eastern fronts quickly opened along the borders of ( Germany, Russia ) and Austria-Hungary.