“The victory over cruelty is never final, but, like the maintenance of freedom, requires eternal vigilance.”

"The Tallest Tale", p. 315
Leonardo's Mountain of Clams and the Diet of Worms (1998)

Address to the United Nations (1963)
Context: The Charter of the United Nations expresses the noblest aspirations of man: abjugation of force in the settlement of disputes between states; the assurance of human rights and fundamental freedoms for all without distinction as to race, sex, language or religion; the safeguarding of international peace and security.
But these, too, as were the phrases of the Covenant, are only words; their value depends wholly on our will to observe and honour them and give them content and meaning. The preservation of peace and the guaranteeing of man's basic freedoms and rights require courage and eternal vigilance: courage to speak and act — and if necessary, to suffer and die — for truth and justice; eternal vigilance, that the least transgression of international morality shall not go undetected and unremedied.
These lessons must be learned anew by each succeeding generation, and that generation is fortunate indeed which learns from other than its own bitter experience. This Organization and each of its members bear a crushing and awesome responsibility: to absorb the wisdom of history and to apply it to the problems of the present, in order that future generations may be born, and live, and die, in peace.

Short story, 'Damaged Goods' - p.60
Short stories, The Turning (2004)

“Color,” p. 64
The Creator (2000), Sequence: “A Smiling Sky”

Source: The Genteel Tradition in American Philosophy (1911), p. 58

This has been attributed to Marshall, and he might have used the phrase, but earlier uses exist:
There is an imperialism that deserves all honor and respect — an imperialism of service in the discharge of great duties. But with too many it is the sense of domination and aggrandisement, the glorification of power. The price of peace is eternal vigilance.
Leonard H. Courtney as quoted in The Life Of Lord Courtney (I920) by G. P. Gooch
Courtney's statement however is probably derived from an earlier statement with several variants:
The price of freedom is eternal vigilance.
The price of liberty is eternal vigilance.
Eternal vigilance is the price of liberty.
These have often been attributed to Thomas Jefferson, but also Thomas Paine, Abraham Lincoln, and many others; Alfred Denning in The Road to Justice (1988) states that the phrase originated in a statement of Irish orator John Philpot Curran in 1790: "It is the common fate of the indolent to see their rights become a prey to the active. The condition upon which God hath given liberty to man is eternal vigilance."
Misattributed

[Chaturvedi, Pradeep, Women and Food Security: Role of Panchayats, http://books.google.com/books?id=IuKV5ak57asC&pg=PA46, 1 January 2002, Concept Publishing Company, 978-81-7022-873-8, 46–]

From Does Price Fixing Destroy Liberty? (1920) by George H. Earle, Jr.