Which military action led to the start of world war 1

While it was not truly an military action, it was mostly a political action, with the start of Austio-Hungary trying to annex Serbia, leading to the assassination of the leaders of Austrio-Hungary Archduke Ferdinand and his wife Sophie by a Serb Princip, who was against the annexation.

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hello there the answer is a

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answer; (d) i believe that the correct answer is americans finally ran out of ammunition;