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The German violation of Belgian neutrality (invasion of Belgium).
The question as posed does not really make sense. It was the Russian defense of Serbia against Austria-Hungary's invasion that started World War 1. The German declaration of war against Russia then triggered the alliance of the Triple Entente, expanding the reach of the conflict. Once the Triple Entente was involved, the Germans wanted to invade France. But in order to do so and outflank the French forces on the German/French border, they had to invade the north of France, going first through Belgium. So Belgium was the first country to be invaded.
Russia was the country widen the Austro-Serbian conflict into a full-scale regional war when it came to little Serbia's defense against empiric Austria-Hungary. France joined the war because of its alliance with Russia and the projected German invasion. Britain came in when Belgium was invaded by Germany, who violated a treaty that compelled Britain's defense of Belgium in such a case.
The German invasion of Belgium was the official reason for Britain joining the war.
Britain entered the war as a result of the German invasion of Belgium, as Britain had a defense treaty with Belgium (Britain wanted to enter the war and found a 100 year old treaty to use as pretext). Italy, who was allied with Germany before the war, did not enter on the German side as they had a defense pact, and joined with the Entente powers with promises of German and Austrian colonies and territory.