On 4 August 1914, Britain declared war on Germany. People in Britain opened their newspapers the next morning and found out war had begun.
It became known as 'The Great War' because it affected people all over the world and was the biggest war anyone had ever known. The war was fought between two powerful groups.
The opposing sides were:
Germany, Austria-Hungary and Italy who were known as 'The Triple Alliance'.Great Britain, France and Russia who were known as 'The Triple Entente'.
Long before the war began, the countries in these groups had made arrangements to work together and help one another if there was a war. So when war did break out in 1914, parts of Europe was already divided into two sides.
Each of the countries involved got their troops ready to fight. Troops were groups that fought together and included both the army and the navy.
There was no single event that caused World War One (WW1). War happened because of several different events that took place in the years building up to 1914.
Firstly, there was the role of empire. Great Britain, Germany, Austria-Hungary and Russia all had empires. This meant that they ruled many countries all over the world.
