As a lawyer, Liebknecht often defended other left-wing socialists who were tried for offences such as smuggling socialist propaganda into Russia, a task in which he was also involved.
He became a member of the SPD in 1900 and was president of the Socialist Youth International from 1907 to 1910.
Liebknecht also wrote extensively against militarism.
In his speech at the Bremen party conference in 1904, he insisted to his audience: "Militarism is our most deadly enemy and the best way of waging the struggle against it is to increase the number of social democrats among the soldiers".
One of his papers, Militarismus und Antimilitarismus (Militarism and Antimilitarism) led to his being arrested in 1907 and imprisoned for eighteen months in Glatz, Prussian Silesia.
In the next year, he was elected to the Prussian parliament despite still being in prison.

Liebknecht was an active member of the Second International and a founder of the Socialist Youth International.
In 1912, Liebknecht was elected to the Reichstag as a Social-Democrat, a member of the SPD's left-wing.
In October that year, he married his second wife, art historian Sophie Ryss.
He opposed Germany's participation in World War I, but in order not to infringe the party's unity he abstained from the vote on war loans on 4 August 1914.
On 2 December 1914, he was the only member of the Reichstag to vote against further loans, the supporters of which included 110 of his own party members.
He continued to be a major critic of the Social-Democratic leadership under Karl Kautsky and its decision to acquiesce in going to war.

At the end of 1914, Liebknecht, together with Rosa Luxemburg, Leo Jogiches, Paul Levi, Ernest Meyer, Franz Mehring and Clara Zetkin, formed the so-called Spartacus League (Spartakusbund).
The Spartacus League publicized its views in a newspaper titled Spartakusbriefe (Spartacus Letters) which was soon declared illegal.
Liebknecht was arrested and sent to the eastern front during World War I despite his immunity as a member of parliament.
Refusing to fight, he served burying the dead and due to his rapidly deteriorating health was allowed to return to Germany in October 1915.

Liebknecht was arrested again following a demonstration against the war in Berlin on 1 May 1916 that was organized by the Spartacus League and sentenced to two and a half years in jail for high treason, which was later increased to four years and one month.
As a lawyer, Liebknecht often defended other left-wing socialists who were tried for offences such as smuggling socialist propaganda into Russia, a task in which he was also involved. He became a member of the SPD in 1900 and was president of the Socialist Youth International from 1907 to 1910. Liebknecht also wrote extensively against militarism. In his speech at the Bremen party conference in 1904, he insisted to his audience: "Militarism is our most deadly enemy and the best way of waging the struggle against it is to increase the number of social democrats among the soldiers". One of his papers, Militarismus und Antimilitarismus (Militarism and Antimilitarism) led to his being arrested in 1907 and imprisoned for eighteen months in Glatz, Prussian Silesia. In the next year, he was elected to the Prussian parliament despite still being in prison.
Liebknecht was an active member of the Second International and a founder of the Socialist Youth International. In 1912, Liebknecht was elected to the Reichstag as a Social-Democrat, a member of the SPD's left-wing. In October that year, he married his second wife, art historian Sophie Ryss. He opposed Germany's participation in World War I, but in order not to infringe the party's unity he abstained from the vote on war loans on 4 August 1914. On 2 December 1914, he was the only member of the Reichstag to vote against further loans, the supporters of which included 110 of his own party members. He continued to be a major critic of the Social-Democratic leadership under Karl Kautsky and its decision to acquiesce in going to war.
At the end of 1914, Liebknecht, together with Rosa Luxemburg, Leo Jogiches, Paul Levi, Ernest Meyer, Franz Mehring and Clara Zetkin, formed the so-called Spartacus League (Spartakusbund). The Spartacus League publicized its views in a newspaper titled Spartakusbriefe (Spartacus Letters) which was soon declared illegal. Liebknecht was arrested and sent to the eastern front during World War I despite his immunity as a member of parliament. Refusing to fight, he served burying the dead and due to his rapidly deteriorating health was allowed to return to Germany in October 1915.
Liebknecht was arrested again following a demonstration against the war in Berlin on 1 May 1916 that was organized by the Spartacus League and sentenced to two and a half years in jail for high treason, which was later increased to four years and one month.
Karl Liebknecht