World War I happened from 1914 to 1918, and it started because of many reasons that brought Britain into the conflict. Here are the main reasons:

1. Militarism:

   • Many countries in Europe were building up their armies and navies.
   • By 1914, Britain had the biggest navy in the world with 20 powerful battleships called Dreadnoughts.
   • Germany was also increasing its naval power to compete with Britain.
   • Because of this focus on military strength, many people believed war was a good way to solve problems.

2. Alliances:

   • A complex network of alliances was created.
   • The major groups involved were the Triple Alliance (Germany, Austria-Hungary, and Italy) and the Triple Entente (France, Russia, and Britain).
   • This meant that if one country had a problem, it could quickly lead to a larger war.
   • For example, when Archduke Franz Ferdinand was killed, Austria-Hungary declared war on Serbia.
   • This action pulled in all the other countries due to their alliance commitments, including Britain.

3. Imperialism:

   • Nations were competing for colonies around the world, which created more tension.
   • By 1914, Britain controlled about 25% of the world’s land and people.
   • Germany wanted to gain more colonies, causing conflicts, especially in places like Africa and Asia.

4. Nationalism:

   • Nationalism made countries feel proud and competitive, especially in Eastern and Southeastern Europe.
   • In these areas, Slavic nationalism was a threat to empires like Austria-Hungary.
   • Britain was also influenced by a sense of national pride, which pushed them to support the Entente powers.

British Involvement:

• All these reasons combined led Britain to declare war on Germany on August 4, 1914.
• This was partly because Germany invaded Belgium, a country Britain promised to protect.
• The British Army quickly prepared for war and sent about 160,000 soldiers at first.
• By the end of the war, around 5 million British soldiers had served.

Because Britain got involved, the effects were huge. About 1 million soldiers died, and 2 million were injured. The war also caused big changes in British society, both socially and economically, after it ended.