Why did the Social Democratic Party of Germany split during World War I?

The Social Democratic Party of Germany (SPD) split during World War I primarily due to internal disagreements over the party's support for the war effort. Specifically, the party's decision to vote in favor of war credits in 1914, combined with the disastrous progression of the conflict, caused a deep fracture within its ranks.1

This internal division resulted in the formation of two distinct splinter groups:

• The Independent Social Democratic Party (USPD): Led by centrists such as Karl Kautsky, this group broke away from the main party due to their opposition to the war policies.1
• The Spartacus League: Led by leftists Rosa Luxemburg and Karl Liebknecht, this faction adopted a more radical stance. In December 1918, this group evolved into the Communist Party of Germany (KPD).1

Following the war, the right wing of the SPD, under Friedrich Ebert, further alienated the radical left by collaborating with military and conservative forces to suppress uprisings, leading to lasting political animosity.12