
Source: 2010s, Why the Left Hates America (2015)
Conclusion
1880s, Personal Memoirs of General U. S. Grant (1885)
Context: The cause of the great War of the Rebellion against the United States will have to be attributed to slavery. For some years before the war began it was a trite saying among some politicians that 'A state half slave and half free cannot exist.' All must become slave or all free, or the state will go down. I took no part myself in any such view of the case at the time, but since the war is over, reviewing the whole question, I have come to the conclusion that the saying is quite true.
Source: 2010s, Why the Left Hates America (2015)
As quoted in "Will the New Congress End U.S. Allegiance to Saudi Arabia and the War in Yemen?", interview by Sharmini Peries, in The Real News https://therealnews.com/stories/will-the-new-congress-end-u-s-allegiance-to-saudi-arabia-and-the-war-in-yemen (6 January 2019)
2019
We would not have been the bastion of freedom we have been in the twentieth century.
2000s, The Real Abraham Lincoln: A Debate (2002), Q&A
“Germ warfare against the United States would escalate to war against all humanity.”
Quoted in "The Second Attack on Pearl Harbor" - Page 201 - by Steve Horn - History - 2005.
2010s, Why the Left Hates America (2015)
Part II: The Banality of Slavery, page 50.
Going Postal: Rage, Murder, and Rebellion, From Reagan's Workplaces to Clinton's Columbine and Beyond (2005)
“Without slavery the rebellion could never have existed; without slavery it could not continue.”
1860s, Second State of the Union address (1862)
"Subduing the Rebellion" (22 January 1862), as quoted in The Selected Works of Thaddeus Stevens http://books.google.com/books?id=A0Fs655TKfsC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
1860s
2000s, The Real Abraham Lincoln: A Debate (2002), Q&A