By when had 11 southern states seceded from the Union?

The 11 southern states had seceded from the Union by the end of 1861. These 11 states, South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, Texas, Virginia, Arkansas, Tennessee, and North Carolina. They broke away from the Union at least in part due to a conflict in belief in slavery, leading to a spiralling conflict which lasted until 1865; ending effectively in April 1865, with a formal end brought by Andrew Johnson's declaration, 20th August 1866.

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The first seven states seceded before the start of the Civil War, with South Carolina being the first to leave. The four other states joined after the conflict had begun, following the Union's attempt to reclaim Fort Sumter in South Carolina. On June 8, 1861, Tennessee became the 11th state to secede from the Union, making June 8, 1861, the answer.