Vandervelde was named Minister of State in 1914 and supported the policy of resistance to the German invasion of Belgium in World War I. As one of the more respected Socialists within Europe, he encouraged other socialists and other parties on the left to support the war against Germany. He wrote a telegram to the socialist party of Russia that was reprinted in several newspapers. In 1916, he entered the de Broqueville government. He was a delegate for Belgium at the Treaty of Versailles and subsequently involved in the League of Nations. In 1923, he helped to found the Labour and Socialist International of which he held the presidency until 1938.