Which event directly caused Britain to declare war on Germany?

German forces declared war on Serbia, which had not attacked any country.
German forces declared war on Russia, which had been Britain's ally.
German forces invaded Belgium, which had been guaranteed neutrality.
German forces invaded France, which was located very close to Britain.

1 answer

The event that directly caused Britain to declare war on Germany was German forces invaded Belgium, which had been guaranteed neutrality. Britain's entry into World War I was initiated on August 4, 1914, when Germany violated Belgium's neutrality as part of its military strategy to invade France. This act prompted Britain to fulfill its obligation to protect Belgium and declare war on Germany.