“An army to be useful must be a unit, and out of this has grown the saying, attributed to Napoleon, but doubtless spoken before the days of Alexander, that an army with an inefficient commander was better than one with two able heads.”

Letter to A.S. Suvorin (October 27, 1888)
Letters

Quoted in "Nanking: Anatomy of an Atrocity" - Page 232 - by Masahiro Yamamoto - History - 2000.

As quoted in Memoirs of General W.T. Sherman, 2nd ed., D. Appleton & Co., 1913 (1889). Reprinted by the Library of America, 1990<!--, ,--> p. 729.
1880s
Context: My aim then was to whip the rebels, to humble their pride, to follow them to their inmost recesses, and make them fear and dread us. 'Fear of the Lord is the beginning of wisdom.' I did not want them to cast in our teeth what General Hood had once done at Atlanta, that we had to call on their slaves to help us to subdue them. But, as regards kindness to the race..., I assert that no army ever did more for that race than the one I commanded at Savannah.

Cardinal Martinez reiterates Pope’s call for peace https://www.catholicnewsagency.com/news/cardinal_martinez_reiterates_popes_call_for_peace (August 9, 2006)

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

Variant translation: Two armies that fight each other is like one large army that commits suicide.
Under Fire (1916), Ch. 1 - The Vision

Source: Down and out in Paris and London (1933), Ch. 7; a remark by Boris

Source: Introduction, p. viii note: 1950s, Fleet Admiral King: A Naval Record (1952)