World War I
The United Kingdom quickly joined the war and said that it was to protect Belgian neutrality. Germany passed through Belgium to reach Paris before Russia could mobilize and open up a second front. On August 4, 1914, the British declared war against Germany in support of Belgium. The British Empire was larger than any other empire and ruled over a quarter of the world. If Germany won in France, it might take British and French colonies and become the most powerful and largest empire in the world.