Presidents quickly realize that while a single act might destroy the world they live in, no one single decision can make life suddenly better or can turn history around for the good.

Lyndon B. Johnson (1908-1973) American politician, educator, US President (1963-69)

Speech, Signing a Bill Extending the Peace Corps Act, Georgetown University (1966-09-13)
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Broader context:

To hunger for use, and to go unused, is the worst hunger of all. [...] It is true that few men have the power by a single act of theirs or in a single lifetime to shape history for themselves. Presidents, for example, quickly realize that while a single act might destroy the world they live in, no one single decision can make life suddenly better or can turn history around all for the good. But Presidents do know that a nation is the sum total of what we all do together; that the deeds and desires of each citizen fashion our character and shape our world -- just as one tiny drop of water after another will ultimately make a mighty river.