The assassination of Franz Ferdinand is what started WW1, which was done by Serbian rebels, but the county was Austria-Hungary. The two nations that escalated the conflict was Russia, who supported Serbia, and Germany, who supported Austria.
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Belgium's ports were close to the British coast and German control of Belgium would have been seen as a serious threat to Britain. Britain refused to ignore the events of 4 August 1914, when Germany attacked France through Belgium. Within hours, Britain declared war on Germany.
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