“A nation that neglects either of these forces perishes. They must be steered together, like a pair of carriage horses.”

On the Irrepressible Conflict (1858)
Context: The Union is a confederation of States. But in another aspect the United States constitute only one nation. Increase of population, which is filling the States out to their very borders, together with a new and extended network of railroads and other avenues, and an internal commerce which daily becomes more intimate, is rapidly bringing the States into a higher and more perfect social unity or consolidation. Thus, these antagonistic systems are continually coming into closer contact, and collision results.
Shall I tell you what this collision means? They who think that it is accidental, unnecessary, the work of interested or fanatical agitators, and therefore ephemeral, mistake the case altogether. It is an irrepressible conflict between opposing and enduring forces, and it means that the United States must and will, sooner or later, become either entirely a slaveholding nation, or entirely a free-labor nation.

Source: The Portrait of a Lady

Violinist Yehudi Menuhin

Attributed to John Mair, not John Muir, in Toasts and Tributes, edited by Arthur Gray (Rohde and Haskins, New York, 1904) page 154.
Misattributed

Achtung-Panzer! : The Development of Armoured Forces, Their Tactics and Operational Potential (1937)

A New System of Sword Exercise for Infantry (1876)
Context: The recruit must be carefully and sedulously taught when meeting the enemy, even at a trot or canter, to use no force whatever, otherwise his sword will bury itself to the hilt, and the swordsman will either be dragged from his horse, or will be compelled to drop his weapon — if he can. Upon this point I may quote my own System of Bayonet Exercise (p. 27): —
"The instructor must spare no pains in preventing the soldier from using force, especially with the left or guiding arm, as too much exertion generally causes the thrust to miss. A trifling body-stab with the bayonet (I may add with the sword) is sufficient to disable a man; and many a promising young soldier has lost his life by burying his weapon so deep in the enemy's breast that it could not be withdrawn quickly enough to be used against a second assailant. To prevent this happening, the point must be delivered smartly, with but little exertion of force, more like a dart than a thrust, and instantly afterwards the bayonet must be smartly withdrawn." In fact the thrust should consist of two movements executed as nearly simultaneously as possible; and it requires long habit, as the natural man, especially the Englishman, is apt to push home, and to dwell upon his slouching push.

Opening address, Pacific Islands Political Studies Association (PIPSA), 24 November 2005.

Source: Liberalism (1927), Ch. 1 : The Foundations of Liberal Policy § 10 : The Argument of Fascism
Context: Repression by brute force is always a confession of the inability to make use of the better weapons of the intellect — better because they alone give promise of final success. This is the fundamental error from which Fascism suffers and which will ultimately cause its downfall. The victory of Fascism in a number of countries is only an episode in the long series of struggles over the problem of property. The next episode will be the victory of Communism. The ultimate outcome of the struggle, however, will not be decided by arms, but by ideas. It is ideas that group men into fighting factions, that press the weapons into their hands, and that determine against whom and for whom the weapons shall be used. It is they alone, and not arms, that, in the last analysis, turn the scales.
So much for the domestic policy of Fascism. That its foreign policy, based as it is on the avowed principle of force in international relations, cannot fail to give rise to an endless series of wars that must destroy all of modern civilization requires no further discussion. To maintain and further raise our present level of economic development, peace among nations must be assured. But they cannot live together in peace if the basic tenet of the ideology by which they are governed is the belief that one's own nation can secure its place in the community of nations by force alone.
It cannot be denied that Fascism and similar movements aiming at the establishment of dictatorships are full of the best intentions and that their intervention has, for the moment, saved European civilization. The merit that Fascism has thereby won for itself will live on eternally in history. But though its policy has brought salvation for the moment, it is not of the kind which could promise continued success. Fascism was an emergency makeshift. To view it as something more would be a fatal error.