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Heir to the Austrian throne was murdered by the Serb's Black Hand, and Austria declared war on Serbia.
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Russia declared war on Austria in Serbia's defense, and started mobilizing their massive army
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Austria, dissatisfied with Serbia's response to their letter, declares war on Serbia.
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Germany, bound by a treaty with Austria, declared war on Russia
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France, because of a treaty with Russia, had to defend itself from Germany, forcing it to enter the war.
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Placed my moral obligation to defend France, Britain declared war on Germany.
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Britain's colonies, Australia, Canada, India, New Zealand and the Union of South Africa, all entered the war with Britain.
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Because of a treaty with England, Japan had to enter the war to defend it.
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Italy said that Germany were taking offense, and it only had to help if they were on defense, making it declare neutrality. Now, it entered the war against Germany and Austria, it's two former allies.
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Woodrow Wilson declared the US neutral, until German U-boats forced to destroy all shipping from the US, forcing the US to enter the war or go bankrupt.