“We know it because democracy alone, of all forms of government, enlists the full force of men's enlightened will.”

26 June 1787 per page 105 of "The Debates, Resolutions, and Other Proceedings, in Convention, on the Adoption of the Federal Constitution: Supplementary to the state Conventions" by Johnathan Elliot, published 1830 https://books.google.ca/books?id=-gtAAAAAYAAJ&pg=RA1-PA105
Debates of the Federal Convention (1787)

What is Patriotism? (1908)
Context: Our writer claims that militarism can never become such a power in America as abroad, since it is voluntary with us, while compulsory in the Old World. Two very important facts, however, the gentleman forgets to consider. First, that conscription has created in Europe a deep-seated hatred of militarism among all classes of society. Thousands of young recruits enlist under protest and, once in the army, they will use every possible means to desert. Second, that it is the compulsory feature of militarism which has created a tremendous anti-militarist movement, feared by European Powers far more than anything else. After all, the greatest bulwark of capitalism is militarism. The very moment the latter is undermined, capitalism will totter. True, we have no conscription; that is, men are not usually forced to enlist in the army, but we have developed a far more exacting and rigid force--necessity. Is it not a fact that during industrial depressions there is a tremendous increase in the number of enlistments? The trade of militarism may not be either lucrative or honorable, but it is better than tramping the country in search of work, standing in the bread line, or sleeping in municipal lodging houses. After all, it means thirteen dollars per month, three meals a day, and a place to sleep. Yet even necessity is not sufficiently strong a factor to bring into the army an element of character and manhood. No wonder our military authorities complain of the "poor material" enlisting in the army and navy. This admission is a very encouraging sign. It proves that there is still enough of the spirit of independence and love of liberty left in the average American to risk starvation rather than don the uniform.

Speech in the House of Commons (11 November 1947), published in 206–07 The Official Report, House of Commons (5th Series), 11 November 1947, vol. 444, cc. http://hansard.millbanksystems.com/commons/1947/nov/11/parliament-bill#column_206
Post-war years (1945–1955)
Variant: Democracy is the worst form of government, except all the others that have been tried.
Context: Many forms of Government have been tried and will be tried in this world of sin and woe. No one pretends that democracy is perfect or all-wise. Indeed, it has been said that democracy is the worst form of government except all those other forms that have been tried from time to time.

1940s, Third inaugural address (1941)

Source: Born to be Riled (1999), p. 34

1940s, Third inaugural address (1941)

What is Enlightenment? (1784)