Treaty of London
Britain's reasons for declaring war were complex. After the war began the propaganda reason given was that Britain was required to safeguard Belgium's neutrality under the Treaty of London 1839. The German invasion of Belgium was, therefore, the "casus belli" and, importantly, legitimized and galvanized popular support for the war among the antiwar Liberal Party constituency. However, the Treaty of London of 1839 had not committed Britain on her own to safeguard Belgium's neutrality.