I have always been fond of the West African proverb “Speak softly and carry a big stick; you will go far.”

Theodore Roosevelt (1858-1919) American politician, statesman, conservationist, writer, US President (1901-1909)

Letter (1900-01-26) to Henry L. Sprague
    (Source)

The letter to Sprague is is the first known use by Roosevelt of his future catch phrase.  It attained more fame when he used it in a speech at the Minnesota State Fair (1901-09-02) (two versions found):

There is a homely adage which runs "Speak softly and carry a big stick; you will go far." If the American nation will speak softly and yet build and keep at a pitch of highest training a thoroughly efficient Navy, the Monroe Doctrine will go far.
[Speech (1901-09-02), "A Nation of Pioneers," Minnesota State Fair, Minneapolis]

A good many of you are probably acquainted with the old proverb "Speak softly and carry a big stick -- you will go far."
[Minneapolis Tribune (1901-09-03)]

Other significant references:

Right here let me make as vigorous a plea as I know how in favor of saying nothing that we do not mean, and of acting without hesitation up to whatever we say. A good many of you are probably acquainted with the old proverb, 'Speak softly and carry a big stick -- you will go far.' If a man continually blusters, if he lacks civility, a big stick will not save him from trouble, and neither will speaking softly avail, if back of the softness there does not lie strength, power.
[Speech (1903-04-02), Chicago]

One the main lessons to learn from this war is embodied in the homely proverb: "Speak softly and carry a big stick."
[America and the World War, ch. 2 "The Belgian Tragedy" (1915)]

The only safe rule [in foreign policy] is to promise little, and faithfully keep to every promise; to "speak softly and carry a big stick."
[Theodore Roosevelt: An Autobiography, ch. 15 "The Peace of Righteousness" (1913)]

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