“We must seize the once-in-a-lifetime opportunity offered by the end of free movement.”

2000s, 2002, State of the Union address (January 2002)

Said in an interview with Sveriges Radio - Ekot about the EU enlargement (November 11, 2003).

Source: Give Me Liberty! (1998), Ch. 7 : The New Slave Master, p. 89

Sun Tzu among many other military thinkers and leaders believed in fate and determination from the correct application of theory, the state of the opponent's and one's own power, and a code for the general and a code for the soldier to follow, rather than the Machiavellian type of intuition that evokes an evolution of opportunism that brought great historical consequences as it dominated over the classical and medieval ethical doctrines. Thus this statement is contrary to Sun Tzu principles. Nevertheless, there is a possible relation to the quote: Quickness is the essence of the war.
Misattributed

1980s, Second term of office (1985–1989), Farewell Address (1989)
Context: The lesson of all this was, of course, that because we're a great nation, our challenges seem complex. It will always be this way. But as long as we remember our first principles and believe in ourselves, the future will always be ours. And something else we learned: Once you begin a great movement, there's no telling where it will end. We meant to change a nation, and instead, we changed a world.

Out of Control: The New Biology of Machines, Social Systems and the Economic World (1995), New Rules for the New Economy: 10 Radical Strategies for a Connected World (1999)

Section 7
The True Believer (1951), Part One: The Appeal of Mass Movements
Context: There is a fundamental difference between the appeal of a mass movement and the appeal of a practical organization. The practical organization offers opportunities for self-advancement, and its appeal is mainly to self-interest. On the other hand, a mass movement, particularly in its active, revivalist phase, appeals not to those intent on bolstering and advancing a cherished self, but to those who crave to be rid of an unwanted self. A mass movement attracts and holds a following not because it can satisfy the desire for self-advancement, but because it can satisfy the passion for self-renunciation.

Source: EU referendum result must be respected, says John McDonnell https://www.bbc.co.uk/news/uk-politics-36680463 BBC News (1 July 2016)

Source: Manuscript Found in Accra (2012), The Defeated Ones