* Britain had a treaty with Belgium that obligated it to defend Belgium if it was invaded. When Germany invaded Belgium in August 1914, Britain declared war on Germany.
* Britain also had a treaty with France, which obligated Britain to come to France's aid if it was attacked. France was attacked by Germany in August 1914, so Britain declared war on Germany.
* Britain's economy was also closely linked to the economies of its allies, particularly France and Belgium. If these countries were defeated by Germany, Britain's economy would suffer.
To prevent Germany from becoming too powerful
* Germany was a rapidly rising power in the early 20th century. It was building a powerful navy, and its army was one of the most powerful in the world. If Germany was not stopped, it could become a threat to Britain's security and its empire.
To uphold the balance of power in Europe
* Britain had traditionally played a role in maintaining the balance of power in Europe. If Germany was allowed to become too powerful, it could upset the balance of power and lead to war.
To preserve the British Empire
* The British Empire was one of the most important global powers in the early 20th century. If Britain was defeated by Germany, its empire could be lost.