Historians writing about the origins of World War I have differed over the relative emphasis they place upon the factors involved. Changes in historical arguments over time are in part related to the delayed availability of classified historical archives. The deepest distinction among historians remains between those who focus on the actions of Germany and Austria-Hungary as key and those who focus on a wider group of actors. Meanwhile some historians, such as Fritz Fischer, maintain that Germany deliberately sought war while others do not. The main distinction among the latter is between those who believe that a war between the "Great Powers" was ultimately unplanned but still caused principally by Germany and Austria-Hungary taking risks, and those who believe that either all or some of the other powers, namely Russia, France, Serbia and Great Britain, played a more significant role in risking war than had been traditionally suggested.

The Russian Empire's entry into World War I unfolded gradually in the days leading up to July 28, 1914. The sequence of events began with Austria-Hungary's declaration of war on Serbia, a Russian ally. In response, Russia issued an ultimatum to Vienna via Saint Petersburg, warning Austria-Hungary against attacking Serbia. As the conflict escalated with the invasion of Serbia, Russia commenced mobilizing its reserve army along the border of Austria-Hungary. Consequently, on July 31, Germany demanded that Russia demobilize. When Russia did not comply, Germany declared war on Russia on August 1, 1914. According to its war plan, Germany prioritized its offensive against France, declaring war on August 3. Germany deployed its main armies through Belgium with the aim of encircling Paris. The imminent threat to Belgium prompted Britain to declare war on Germany on August 4. The Ottoman Empire subsequently joined the Central Powers and engaged in warfare against Russia along their shared border.