All modern speeches owe their greatness to one source

And that would be Lincoln's Gettysburg Address. No other speech is as recognizable for its clarity, strength, and symbolism as Lincoln's two-minute homily from 1863. That's why so many politicians like to borrow words, phrases, and even the cadence from that speech. Heck, even I used it as the basis for my campaign talk while running for National Honor Society president in high school. And it worked. I won.
President Obama followed the trend on June 5 when he spoke at the Buchenwald concentration camp in Germany. Here is part of his speech:

And it is now up to us, the living, in our work, wherever we are, to resist injustice and intolerance and indifference in whatever forms they may take, and ensure that those who were lost here did not go in vain. (2009)

Which sounds a lot like this section by Lincoln:

It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we here highly resolve that these dead shall not have died in vain. (1863)

And then Obama cherry-picked this familiar phrasing as well:

It is up to us to redeem that faith. It is up to us to bear witness; to ensure that the world continues to note what happened here; (2009)

As Lincoln said 146 years earlier:

The world will little note, nor long remember what we say here, but it can never forget what they did here. (1863)

I bet $100 that Obama's chief speechwriter, Jon Favreu, has a well-thumbed copy of Lincoln: Speeches and Writings Vol. 2 in his suitcase. Or because he's a young guy, saved on his iPhone.