The American Civil War by one vote

Could the change of one vote in Congress have obviated the Civil War? A distinguished historian thinks so. Francis W. Hirst, in his "Life and Letters of Thomas Jefferson," thus tells the story:

"In 1784 he [Jefferson] was chairman of a committee appointed by Congress to devise a plan of government for the western territories above the parallel of degrees N. Lat. embracing the territory which was afterwards converted into the states of Alabama, Mississippi, Tennessee and Kentucky. The report drafted by Jefferson [March 1, 1784] provided that 'after the year 1800 of the Christian era there shall be neither slavery nor involuntary servitude in any of the said states.'

"This admirable clause, which would have stopped the growth of the slave power and would almost certainly have obviated the Civil War, was lost by one vote. It is one of the tragedies of American history. As Jefferson himself wrote soon afterwards: 'The voice of a single individual would have prevented this abominable crime [of slavery] from spreading itself over the new country. Thus we see the fate of millions unborn hanging on the tongue of one man, and Heaven was silent in that awful moment. But it is to be hoped that it will not always be silent, and that the friends to the rights of human nature will in the end prevail.' "

Source: “American Politics” by the Lincoln Financial Foundation Collection

This is what is later known as the “Jeffersonian Proviso.”