“We in this country, in this generation, are — by destiny rather than choice — the watchmen on the walls of world freedom. We ask, therefore, that we may be worthy of our power and responsibility, that we may exercise our strength with wisdom and restraint, and that we may achieve in our time and for all time the ancient vision of "peace on earth, good will toward men". That must always be our goal, and the righteousness of our cause must always underlie our strength.”

1960s, October surprise speech (1968)

"What We Owe Our Parasites", speech (June 1968); Free Speech magazine (October and November 1995)
1960s

Report on the Potsdam Conference (1945)
Context: Any man who sees Europe now must realize that victory in a great war is not something you win once and for all, like victory in a ball game. Victory in a great war is something that must be won and kept won. It can be lost after you have won it — if you are careless or negligent or indifferent.
Europe today is hungry. I am not talking about Germans. I am talking about the people of the countries which were overrun and devastated by the Germans, and particularly about the people of Western Europe. Many of them lack clothes and fuel and tools and shelter and raw materials. They lack the means to restore their cities and their factories.
As the winter comes on, the distress will increase. Unless we do what we can to help, we may lose next winter what we won at such terrible cost last spring. Desperate men are liable to destroy the structure of their society to find in the wreckage some substitute for hope. If we let Europe go cold and hungry, we may lose some of the foundations of order on which the hope for worldwide peace must rest.
We must help to the limits of our strength. And we will.

Can Love be controlled by Advice?, reported in Bartlett's Familiar Quotations, 10th ed. (1919).

2000s, 2001, First inaugural address (January 2001)
Context: Together, we will reclaim America’s schools, before ignorance and apathy claim more young lives. We will reform Social Security and Medicare, sparing our children from struggles we have the power to prevent. And we will reduce taxes, to recover the momentum of our economy and reward the effort and enterprise of working Americans. We will build our defenses beyond challenge, lest weakness invite challenge. We will confront weapons of mass destruction, so that a new century is spared new horrors. The enemies of liberty and our country should make no mistake: America remains engaged in the world by history and by choice, shaping a balance of power that favors freedom. We will defend our allies and our interests. We will show purpose without arrogance. We will meet aggression and bad faith with resolve and strength. And to all nations, we will speak for the values that gave our nation birth.

2011, Address on interventions in Libya (March 2011)

As quoted by Raymond Lonergan in Mr. Justice Brandeis, Great American (1941), p. 42.
Extra-judicial writings

Letter to http://memory.loc.gov/cgi-bin/query/r?ammem/mtj:@field(DOCID+@lit(tj110158)) Thomas Leiper (12 June 1815). Published in The Works of Thomas Jefferson in Twelve Volumes http://oll.libertyfund.org/ToC/0054.php, Federal Edition, Paul Leicester Ford, ed., New York: G. P. Putnam's Sons, 1904, Vol. 11 http://oll.libertyfund.org/Texts/Jefferson0136/Works/0054-11_Bk.pdf, pp. 477–478.
The sentence "I hope our wisdom will grow with our power, and teach us, that the less we use our power, the greater it will be." was used by US-President Barack Obama in his A New Beginning Speech.
1810s
Context: We concur in considering the government of England as totally without morality, insolent beyond bearing, inflated with vanity and ambition, aiming at the exclusive dominion of the sea, lost in corruption, of deep-rooted hatred towards us, hostile to liberty wherever it endeavors to show its head, and the eternal disturber of the peace of the world. In our estimate of Bonaparte, I suspect we differ. [... ] Our form of government is odious to him, as a standing contrast between republican and despotic rule; and as much from that hatred, as from ignorance in political economy, he had excluded intercourse between us and his people, by prohibiting the only articles they wanted from us, that is, cotton and tobacco. Whether the war we have had with England, and the achievements of that war, and the hope that we may become his instruments and partisans against that enemy, may induce him, in future, to tolerate our commercial intercourse with his people, is still to be seen. For my part, I wish that all nations may recover and retain their independence; that those which are overgrown may not advance beyond safe measures of power, that a salutary balance may be ever maintained among nations, and that our peace, commerce, and friendship, may be sought and cultivated by all. It is our business to manufacture for ourselves whatever we can, to keep our markets open for what we can spare or want; and the less we have to do with the amities or enmities of Europe, the better. Not in our day, but at no distant one, we may shake a rod over the heads of all, which may make the stoutest of them tremble. But I hope our wisdom will grow with our power, and teach us, that the less we use our power, the greater it will be.

Armistice Day speech (11 November 1948), published in Omar Bradley's Collected Writings, Volume 1 (1967)