“United States: …. A nation given either to unjustified over-enthusiasms or infantile furies.”

"Niagara Movement Speech" (1905) http://teachingamericanhistory.org/library/document/niagara-movement-speech/ <!--originally a portion of this was cited here to an Address to the Nation speech at Harpers Ferry, Virginia (16 August 1906); published in the New York Times on (20 August 1906) — but that does not correspond with the info at the link. -->
Context: The school system in the country districts of the South is a disgrace and in few towns and cities are Negro schools what they ought to be. We want the national government to step in and wipe out illiteracy in the South. Either the United States will destroy ignorance or ignorance will destroy the United States.
And when we call for education we mean real education. We believe in work. We ourselves are workers, but work is not necessarily education. Education is the development of power and ideal. We want our children trained as intelligent human beings should be, and we will fight for all time against any proposal to educate black boys and girls simply as servants and underlings, or simply for the use of other people. They have a right to know, to think, to aspire.
These are some of the chief things which we want. How shall we get them? By voting where we may vote, by persistent, unceasing agitation; by hammering at the truth, by sacrifice and work.
We do not believe in violence, neither in the despised violence of the raid nor the lauded violence of the soldier, nor the barbarous violence of the mob, but we do believe in John Brown, in that incarnate spirit of justice, that hatred of a lie, that willingness to sacrifice money, reputation, and life itself on the altar of right. And here on the scene of John Brown’s martyrdom we reconsecrate ourselves, our honor, our property to the final emancipation of the race which John Brown died to make free.
Our enemies, triumphant for the present, are fighting the stars in their courses. Justice and humanity must prevail.

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.

When asked by Viereck if he considered himself to be a German or a Jew. A version with slightly different wording is quoted in Einstein: His Life and Universe http://books.google.com/books?id=dJMpQagbz_gC&lpg=PP1&pg=PA386#v=onepage&q&f=false by Walter Isaacson (2007), p. 386
1920s, Viereck interview (1929)
Variant: Nationalism is an infantile disease. It is the measles of mankind.
Context: It is quite possible to be both. I look upon myself as a man. Nationalism is an infantile disease. It is the measles of mankind.

October 19, 2004 http://www.federalreserve.gov/boarddocs/speeches/2004/20041019/default.htm, playing down the threat of a national housing bubble.
2000s

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.

Zen Masters : The Wisdom of Frank Zappa (2003)

Comments at the centennial celebration of the Lincoln-Douglas debates; Knox College, Galesburg, Illinois, Oct. 7, 1958. Quoted in Herbert Mitgang, "Again—Lincoln v. Douglas", The New York Times Magazine, Oct. 19, 1958, pp. 26-27.
Context: The United States is, not are. The Civil War was fought over a verb. Orval Faubus don't know that. But he gonna know, he gonna know.

1980s, The New View of Reconstruction (1983)

Proclamation against the Nullification Ordinance of South Carolina (11 December 1832)
1830s
Context: To say that any State may at pleasure secede from the Union, is to say that the United States are not a nation because it would be a solecism to contend that any part of a nation might dissolve its connection with the other parts, to their injury or ruin, without committing any offense. Secession, like any other revolutionary act, may be morally justified by the extremity of oppression; but to call it a constitutional right, is confounding the meaning of terms, and can only be done through gross error, or to deceive those who are willing to assert a right, but would pause before they made a revolution, or incur the penalties consequent upon a failure.