Outraged by the German invasion of Belgium:​

The German invasion of Belgium was a military campaign which began on 4 August 1914. Earlier, on 24 July, the Belgian government had announced that if war came it would uphold its historic neutrality. The Belgian government mobilised its armed forces on 31 July and a state of heightened alert (Kriegsgefahr) was proclaimed in Germany. On 2 August, the German government sent an ultimatum to Belgium, demanding passage through the country, and German forces invaded Luxembourg. Two days later, the Belgian government refused the demands and the British government guaranteed military support to Belgium. The German government declared war on Belgium on 4 August, troops crossed the border and began the Battle of Liège.

Explanation:

social contract theory, nearly as old as philosophy itself, is the view that persons' moral and/or political obligations are dependent upon a contract or agreement among them to form the society in which they live