In preparation for the peace conference that was expected to follow World War I, in the spring of 1917 the British Foreign Office established a special section responsible for preparing background information for use by British delegates to the conference. Neutrality of Belgium is Number 29 in a series of more than 160 studies produced by the section, most of which were published after the conclusion of the 1919 Paris Peace Conference. The Kingdom of Belgium was established in 1831, after the southern provinces of the Netherlands successfully revolted and won international recognition of their independence, declared in October 1830. In the Treaty of November 15, 1831, the five Great Powers of Europe—Great Britain, France, Austria, Prussia, and Russia—guaranteed the perpetual neutrality of the new kingdom and the integrity and inviolability of its territory. Originally conceived as a buffer against possible French aggression, a neutral Belgium became one of the pillars of the peaceful European order for the remainder of the 19th century and into the 20th century. In August 1914, at the outset of World War I, Germany invaded and occupied Belgium in violation of the treaty of 1831. This study is a detailed analysis of Belgian neutrality under international law. It refutes arguments advanced by German jurists to justify the invasion. Germany’s violation of Belgian neutrality was a major factor inducing Great Britain to enter the war on the side of France and Russia.