What was the main reason that Britain entered the war? because it was an ally of France
because Germany invaded neutral Belgium
because it supported Slavic nationalism
because Archduke Francis Ferdinand was assassinated

Explanation:

Great Britain entered World War I on 4 August 1914 when the King declared war after the expiration of an ultimatum to Germany. The official explanation focused on protecting Belgium as a neutral country; the main reason, however, was to prevent a French defeat that would have left Germany in control of Western Europe.

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who was the head of the bureau who was the head of the bureau