Why did andrew johnson go against his native state of tennessee when it seceded from the union?

Andrew Johnson went against his native state of Tennessee when it seceded from the Union because he believed that the Confederate secession was not allowed by the Constitution of the United States.

He was the only senator from the South who did not leave his post to secession, and became the most prominent anti-war democrat in the South. In 1862, Lincoln named Johnson as the military governor of Tennessee, where he demonstrated dynamism and efficiency in the fight against rebellion. His policy of reconciliation to the South, his haste to reincorporate former Confederates back into the US Union, and his vetoes of civil rights bills involved him in a bitter dispute with Republicans. The Republicans in the House of Representatives tried to impose criminal charges against him in 1868, but he was acquitted by a single vote in the Senate.

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