“An army of one answers to none.”

Army of Occupation (1866), a Civil War poem written at Arlington, Virginia.

Source: The Art of War, Chapter XIII · Intelligence and Espionage

Morning in the Burned House (1995), The Loneliness of the Military Historian
Context: Despite the propaganda, there are no monsters,
or none that can be finally buried.
Finish one off, and circumstances
and the radio create another.
Believe me: whole armies have prayed fervently
to God all night and meant it,
and been slaughtered anyway.
Brutality wins frequently,
and large outcomes have turned on the invention
of a mechanical device, viz. radar.
True, valour sometimes counts for something,
as at Thermopylae. Sometimes being right —
though ultimate virtue, by agreed tradition,
is decided by the winner.
Sometimes men throw themselves on grenades
and burst like paper bags of guts
to save their comrades.
I can admire that.
But rats and cholera have won many wars.
Those, and potatoes,
or the absence of them.

Canto XXVII, lines 61–66 (tr. Sinclair).
The Divine Comedy (c. 1308–1321), Inferno

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

Such an army will be invincible....
On Protracted Warfare (1938)

Speech on the war in French Algeria before French National Assembly (1957), cited in Torture: The Role of Ideology in the French–Algerian War (1989) by Rita Maran, p. 44

(zh-CN) 军队须和民众打成一片，使军队在民众眼睛中看成是自己的军队，这个军队便无敌于天下，个把日本帝国主义是不够打的。
1930s, On Protracted Warfare (1938)