World war I was one most brutal to have happened in history. The German empire was determined to control Belgium, which bordered Britain. They also less expected Britain's participation in the war. The German hoped for disregard for signing the London treaty, which would defend Belgium. The British were suspicious of Germany's quest and therefore opted to declare war against them.
Why did Britain go to war? Background | The National Archives. Nationalarchives.gov.uk. (2020). Retrieved 25 September 2020, from https://www.nationalarchives.gov.uk/education/greatwar/g2/backgroundcs1.htm.