Before the First World War the Social Democratic Party (SPD) was the largest party in Germany and the world's most successful socialist party. Although still officially claiming to be a Marxist party, by 1914 it had become in practice a reformist party. In 1914 the SPD members of the Reichstag voted in favour of the war. Left-wing members of the party, led by Karl Liebknecht and Rosa Luxemburg, strongly opposed the war, and the SPD soon suffered a split, with the leftists forming the Independent Social Democratic Party of Germany (USPD) and the more radical Spartacist League. In November 1918, revolution broke out across Germany. The leftists, led by Rosa Luxemburg and the Spartacist League, formed the KPD at a founding congress held in Berlin on 30 December 1918 – 1 January 1919 in the reception hall of the City Council. Apart from the Spartacists, another dissent group of Socialists called the International Communists of Germany, also dissenting members of the Social Democratic party, but mainly located in Hamburg, Bremen and Northern Germany, joined the young party. The Revolutionary Shop Stewards, a network of dissenting socialist trade unionists centered in Berlin were also invited to the Congress, but eventually did not join the party because they deemed the founding congress leaning into a syndicalist direction.