“Since you cannot mix wolves with sheep, why would anyone dare to compare a lion with the lamb?”

Source: Black: The Birth of Evil

Source: The Prince (1513), Ch. 18
Variant translations of portions of this passage:
Every one admits how praiseworthy it is in a prince to keep faith, and to live with integrity and not with craft. Nevertheless our experience has been that those princes who have done great things have held good faith of little account, and have known how to circumvent the intellect of men by craft, and in the end have overcome those who have relied on their word.
Ch. 18. Concerning the Way in which Princes should keep Faith (as translated by W. K. Marriott)
A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from traps, and the fox cannot defend himself from wolves. One must therefore be a fox to recognize traps, and a lion to frighten wolves.
You must know there are two ways of contesting, the one by the law, the other by force; the first method is proper to men, the second to beasts; but because the first is frequently not sufficient, it is necessary to have recourse to the second.
Context: A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from snares, and the fox cannot defend himself from wolves. One must therefore be a fox to recognise snares, and a lion to frighten wolves. Those that wish to be only lions do not understand this.
Context: How laudable it is for a prince to keep good faith and live with integrity, and not with astuteness, every one knows. Still the experience of our times shows those princes to have done great things who have had little regard for good faith, and have been able by astuteness to confuse men's brains, and who have ultimately overcome those who have made loyalty their foundation. You must know, then, that there are two methods of fighting, the one by law, the other by force: the first method is that of men, the second of beasts; but as the first method is often insufficient, one must have recourse to the second. It is therefore necessary to know well how to use both the beast and the man. This was covertly taught to princes by ancient writers, who relate how Achilles and many others of those princes were given to Chiron the centaur to be brought up, who kept them under his discipline; this system of having for teacher one who was half beast and half man is meant to indicate that a prince must know how to use both natures, and that the one without the other is not durable. A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from snares, and the fox cannot defend himself from wolves. One must therefore be a fox to recognise snares, and a lion to frighten wolves. Those that wish to be only lions do not understand this. Therefore, a prudent ruler ought not to keep faith when by so doing it would be against his interest, and when the reasons which made him bind himself no longer exist. If men were all good, this precept would not be a good one; but as they are bad, and would not observe their faith with you, so you are not bound to keep faith with them.... those that have been best able to imitate the fox have succeeded best. But it is necessary to be able to disguise this character well, and to be a great feigner and dissembler.

The Iliad of Homer, Rendered into English Prose (1898), Book XXII

Book XXII, lines 287–289; spoken by Achilles.
Translations, Iliad (1997)

Source: Scapegoat: The Jews, Israel, and Women's Liberation (2000), pp. 245–246.

Source: Darkfever

Letter to Thomas Cushing (1773) http://oll.libertyfund.org/titles/franklin-the-works-of-benjamin-franklin-vol-vi-letters-and-misc-writings-1772-1775#lf1438-06_head_007.
Context: But our great security lies, I think, in our growing strength, both in numbers and wealth; … unless, by a neglect of military discipline, we should lose all martial spirit …; for there is much truth in the Italian saying, Make yourselves sheep, and the wolves will eat you.

Attributed to Napoleon in Napoleon (1941) by Yevgeny Tarle, this is a variant of an ancient proverb often attributed to many military and political figures, including Alexander the Great, and the even earlier figure Chabrias (Χαβρίας).
Misattributed

Attributed to Alexander, as quoted in The British Battle Fleet: Its Inception and Growth Throughout the Centuries to the Present Day (1915) by Frederick Thomas Jane, but many variants of similar statements exist which have been attributed to others, though in research done for Wikiquote definite citations of original documents have not yet been found for any of them:
I should prefer an army of stags led by a lion, to an army of lions led by a stag.
Attributed to Chabrias, who died around the time Alexander was born, thus his is the earliest life to whom such assertions have been attributed; as quoted in A Treatise on the Defence of Fortified Places (1814) by Lazare Carnot, p. 50
An army of stags led by a lion would be better than an army of lions led by a stag.
Attributed to Chabrias, A History of Ireland (1857) by Thomas Mooney, p. 760
An army of stags led by a lion is superior to an army of lions led by a stag.
Attributed to Chabrias, The New American Cyclopaedia : A Popular Dictionary of General Knowledge (1863), Vol. 4, p. 670
An army of sheep led by a lion are more to be feared than an army of lions led by a sheep.
Attributed to Chabrias, The Older We Get, The Better We Were, Marine Corps Sea Stories (2004) by Vince Crawley, p. 67
It is better to have sheep led by a lion than lions led by a sheep.
Attributed to Polybius in Between Spenser and Swift: English Writing in Seventeenth Century Ireland (2005) by Deana Rankin, p. 124, citing A Contemporary History of Affairs in Ireland, from 1641 to 1652 (1880) by John Thomas Gilbert Vol. I, i, p. 153 - 157; but conceivably this might be reference to Polybius the historian quoting either Alexander or Chabrias.
An army composed of sheep but led by a lion is more powerful than an army of lions led by a sheep.
"Proverb" quoted by Agostino Nifo in De Regnandi Peritia (1523) as cited in Machiavelli - The First Century: Studies in Enthusiasm, Hostility, and Irrelevance (2005) by Mathew Thomson, p. 55
Greater is an army of sheep led by a lion, than an army of lions led by a sheep.
Attributed to Daniel Defoe (c. 1659 - 1731)
I am more afraid of one hundred sheep led by a lion than one hundred lions led by a sheep.
Attributed to Charles Maurice de Talleyrand-Périgord (1754 – 1838) Variants: I am more afraid of an army of 100 sheep led by a lion than an army of 100 lions led by a sheep.
I am not afraid of an army of one hundred lions led by a sheep. I am afraid of army of 100 sheeps led by a lion.
Variants quoted as an anonymous proverb:
Better a herd of sheep led by a lion than a herd of lions led by a sheep.
A flock of sheep led by a lion was more powerful than a flock of lions led by a sheep.
An army of sheep led by a lion would defeat an army of lions led by a sheep.
It were better to have an army of sheep led by a lion than an army of lions led by a sheep.
An army of sheep led by a lion, will defeat an army of lions led by a sheep.
An army of sheep led by a lion would be superior to an army of lions led by a sheep.
Unsourced attribution to Alexander: I would not fear a pack of lions led by a sheep, but I would always fear a flock of sheep led by a lion.
As one lion overcomes many people and as one wolf scatters many sheep, so likewise will I, with one word, destroy the peoples who have come against me.
This slightly similar statement is the only quote relating to lions in The History of Alexander the Great, Being the Syriac Version of the Pseudo-Callisthenes (1889) as translated by E. A. Wallis Budge, but it is attributed to Nectanebus (Nectanebo II).
Disputed