Liebknecht's father was Wilhelm Liebknecht an ethnic German who helped form the Social Democratic Party of Germany in 1891. His mother was Natalia Reh, generally held to have been a Jewess. In 1912 Karl was elected into Reichstag on the left-wing ballot SDP. He was also against Germany's involvement in the First World War and in the end of 1914 he had joined up with other Communists, Rosa Luxemburg, Leo Jogiches, Paul Levi, Ernest Meyer, Franz Mehring and Clara Zetkin and formed the Socialist Newspaper, Spartacus League.
In January, 1915, Liebknecht, like the Bolsheviks in Russia, began arguing that socialists should turn this Nationalist conflict into a revolutionary war. He was arrested and then conscripted into the German Army. Refusing to fight, Liebknecht served on the Eastern Front burying the dead. His health deteriorated and in October, 1915, he was allowed to return to Germany.On 1st May, 1916, the Spartacus League decided to come out into the open and organized a demonstration against the First World War in Berlin. Several of its leaders, including Liebknecht were arrested and imprisoned. They were not released until October, 1918, when Max von Baden granted an amnesty to all political prisoners.
In January, 1919, Liebknecht joined with Rosa Luxemburg, Leo Jogiches and Clara Zetkin in the Spartakist Rising (German Revolution) that took place in Berlin. Friedrich Ebert, the leader of the Social Democrat Party and Germany's new chancellor, called in the German Army and the Freikorps to bring an end to the rebellion. By 13th January the rebellion had been crushed and most of its leaders, including Liebknecht were arrested. Karl Liebknecht was executed on January 15, 1919.