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Russia mobilizes its forces on Germany's eastern borders....
The event that led to Germany immediately declaring war on the Allies was Russia moblizing its forces on Germany’s eastern borders.
Why did the Germans declare war in 1914?
In 1914, the Russians made it clear that they would support the Serbians if the latter was attacked by Austria-Hungary.
When Austria-Hungary declared war on Serbia, Russia began to moblize troops even though Germany was against this. Germany then declared war on Russia which was an allied nation.
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Find out more on Russian mobilization at https://brainly.com/question/316727.