The first, because it provides a definitive refutation-once and for all-of the baseless assertion that the South-which, at the time, consisted of only five states-would have irrevocably severed its ties to the other thirteen colonies, had chattel slavery been abolished in the original framing of the U.S. Constitution.
The second, because it illustrates-in enthralling detail-why Abraham Lincoln was the single greatest wartime leader that this nation has ever had.
I don't believe there is a convincing argument to be made that the Southern states would have ratified the Constitution had it freed slaves (assuming it could have purchased them which is a hell of an assumption given the bankrupt Confederation). Hell for that matter that might have defeated it in New York since areas of that state had as high a concentration of slaves as any area of the nation.
It barely passed there as it was.
I do not need convincing of Lincoln's greatness all I have to do is read some of the garbage posted here by the Lincolnhaters such as the latest "Lincoln was Gay" absurdity. But I would not place him above Washington in that regard.