“Many persons believed, or pretended to believe, and confidently asserted, that freed slaves would not make good soldiers; they would lack courage, and could not be subjected to military discipline. Facts have shown how groundless were these apprehensions. The slave has proved his manhood, and his capacity as an infantry soldier, at Milliken's Bend, at the assault upon Port Hudson, and the storming of Fort Wagner. The apt qualifications of the colored man for artillery service have long been known and recognized by the naval service.”
Letter to Abraham Lincoln (5 December 1863); As quoted in Black Union Soldiers in the Civil War, by Hondon B. Hargrove, p. 108
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Edwin M. Stanton 2
American lawyer, judge and politician 1814–1869Related quotes

“I freed a thousand slaves. I could have freed a thousand more if only they knew they were slaves.”
Attributed to Tubman in Dorothy Winbush Riley, My Soul Looks Back 'Less I Forget https://books.google.com/books?id=KpcLAQAAMAAJ&focus=searchwithinvolume&q=%22c.+1865%22 p. 148 (1993). Riley gives a date of "c. 1865" but offers no citation. No source from earlier than 1993 is known. Quoted in Africana: The Encyclopedia of the African and African American Experience (1999) by Henry Louis Gates and Kwame Anthony Appiah, p. 299. Tubman specialists like Jean H. Humez and Kate Clifford Larson deem this one completely spurious. See "Bogus Tubman," by Steve Perisho http://liberlocorumcommunium.blogspot.com/2014/03/bogus-tubman-i-freed-thousands-of.html.<!-- Someone cited this as being in Harriet, The Moses of Her People (1886) by Sarah H. Bradford, but it does not occur in the editions available online. -->
Disputed
Variant: I freed thousands of slaves. I could have freed thousands more, if they had known they were slaves.
“Men who did not know that they were slaves do not know that they have been freed.”
Source: They Thought They Were Free: The Germans, 1933-35 (1955), p. 62; cited in: Quotable Quotes: They Thought They Were Free http://econengineer.wordpress.com/2012/07/01/quotable-quotes-they-thought-they-were-free/ by John@EconEngineer in: The Economical Engineer (1 July 2012)

Source: 1880s, Life and Times of Frederick Douglass (1881), pp. 433–434.
Context: My interviews with President Lincoln and his able Secretary, before narrated, greatly increased my confidence in the anti-slavery integrity of the government, although I confess I was greatly disappointed at my failure to receive the commission promised me by Secretary Stanton. I, however, faithfully believed, and loudly proclaimed my belief, that the rebellion would be suppressed, the Union preserved, the slaves emancipated, and the colored soldiers would in the end have justice done them. This confidence was immeasurably strengthened when I saw Gen. George B. McClellan relieved from the command of the army of the Potomac and Gen. U. S. Grant placed at its head, and in command of all the armies of the United States. My confidence in Gen. Grant was not entirely due to the brilliant military successes achieved by him, but there was a moral as well as military basis for my faith in him. He had shown his single-mindedness and superiority to popular prejudice by his prompt cooperation with President Lincoln in his policy of employing colored troops, and his order commanding his soldiers to treat such troops with due respect. In this way he proved himself to be not only a wise general, but a great man, one who could adjust himself to new conditions, and adopt the lessons taught by the events of the hour. This quality in General Grant was and is made all the more conspicuous and striking in contrast with his West Point education and his former political associations; for neither West Point nor the Democratic party have been good schools in which to learn justice and fair play to the negro.

1777; quoted by Bert L. Vallée, Alcohol in the Western World, Scientific American, Vol. 278, No. 6 (June), 1998, pp. 80-85

Excerpts of Martial law speech (14 December 1981)

Source: 2000s, A New Birth of Freedom: Abraham Lincoln and the Coming of the Civil War (2000), p. 212

Howell Cobb. "Letter to James A. Seddon", in: Encyclopædia Britannica] (1911), Hugh Chisholm, editor, 11th ed., Cambridge University Press.
Quote regarding suggestions that the Confederates turn their slaves into soldiers. Also quoted as 'You cannot make soldiers of slaves, or slaves of soldiers. The day you make a soldier of them is the beginning of the end of the Revolution. And if slaves seem good soldiers, then our whole theory of slavery is wrong'.

2000s, The Real Abraham Lincoln: A Debate (2002), The Right of Secession Is Not the Right of Revolution

“The more a person believes he’s free to think, the more I believe he’s a slave of his thoughts”