Britain was allied with Belgium, and became quickly involved in the war. Britain had promised to protect Belgian neutrality. Germany passed through Belgium to reach Paris before Russia could mobilize and open up a second front against them. On August 4, 1914, Britain declared war against Germany in support of Belgium. Britain had the biggest empire (it ruled over a quarter of the world). If Germany conquered France, it might take Britain and France's colonies and become the most powerful and biggest empire in the world.