What factor immediately caused Britain to enter into World War I? A. Germany invaded the neutral country of Belgium.

B. Austria-Hungary's archduke was assassinated.

C. France entered into an alliance with Russia.

D. Austria-Hungary invaded Serbia.

Option: A. Germany invaded the neutral country of Belgium.

Explanation:

Great Britain stepped into World War I when Germany invaded Belgium. Britain had promised to support Belgium under the Treaty of London of 1839. Belgium was a neutral country which guaranteed protection from Britain. The Germans wanted the British to ignore the treaty and let them pass through Belgium. The British protect Belgium because its ports were close to the British coast. German control of Belgium would have been a threat to Britain. The British wanted to crush Germany's military capability as they entered into the war.

d. migration of french settlers into english colonies