"President Bush promised to bring liberty and freedom to the world."
What is the difference between liberty and freedom?
There is only one word in French, liberté, to express the right to "express any political or religious opinion and live and act without the governement or another country" (OED entry for freedom) or to "live your life in the way that you want, especially without a lot of interference from the governement" (OED entry for liberty). These definitions didn’t clarify the difference in meaning between the two words, so I looked at their etymology :
Free
The primary sense of the adjective is "dear"; the Germanic and Celtic sense comes of it having been applied as the distinctive epithet of those members of the household who were connected by ties of kindred with the head, as opposed to the slaves.
Liberty
Middle English liberte, from Old French, from Latin libertas, from liber, free.
So it seems that freedom has a more political sense, i.e. the right to dispose of your own person without being owned by anyone, where liberty is a more general and individual concept.
The following three articles offer more insight on both words:
An existential approach to the concept of liberty
Liberty and Freedom: The many competing visions of American society
The difference between liberty and freedom
However, I think that this distinction is a very subtle one and most people use these words as synonyms. So why use them together and create a sense of repetition? I think using two different words with a similar meaning helps to reinforce the concept and add some resonance to it. The concept of freedom is very simple and powerful; once it’s out there, there is not much you can add to it, but it’s such a fundamental concept that it feels that one word doesn’t do it justice. "Freedom" is strong, but "Liberty and freedom" feel even more powerful and worth fighting for. Besides, these two words together sound good; maybe that’s due to the fact that they total six syllables, which represent an hemistich, or half-alexandrine, the twelve-syllable verse traditionally used for epic poems in French…