They occurred during President Andrew Johnson's term just one year after the American Civil War ended when the Union defeated the Confederacy.

A Congress consisting of mostly Radical Republicans sat early in the Capitol and aside from the delegation from Tennessee who were allowed in, the few Southern Congressmen who arrived were not seated.

Lincoln and Johnson ran together under the banner of the National Union Party, which brought together Republicans (with the exception of some hard-line abolitionist Radical Republicans who backed John C. Frémont, who eventually dropped out of the race after brokering a deal with Lincoln) and the War Democrats (the minority of Democrats who backed Lincoln's prosecution of the war, as opposed to the Peace Democrats, or Copperheads, who favored a negotiated settlement with the Confederates).

He immediately became embroiled in a dispute with the Radical Republicans over the conditions of Reconstruction; Johnson favored a lenient Reconstruction, while Radical Republicans wanted to continue the military occupation of the South and force Southern states to give freedmen (the newly freed slaves) civil rights (and the right to vote).

Johnson stumped the country in a public speaking tour known as the Swing Around the Circle; he generally supported Democrats but his speeches were poorly received.

As a percentage of the total number of seats available in the House of Representatives, the Republican majority attained in the election of 1866 has never been exceeded in any subsequent Congress.