The Full paragraph reads:
Slowly the barbarous Greeks, the Hellenes, began to erect their own civilization on the ruins of the civilization they had wrecked. ... By the middle of the second millennium the barbarous Achaeans and Dorians had begun to press in on the Aegean world, and by the end of the millennium they had completely conquered it, had destroyed the Aegean civilization, and had imposed their own language on the conquered people. The land relapsed into barbarism. But the curse of every ancient civilization was that its men in the end became unable to fight. Materialism, luxury, safety, even sometimes an almost modern sentimentality, weakened the fibre of each civilized race in turn; each became in the end a nation of pacifists, and then each was trodden under foot by some ruder people that had kept that virile fighting power the lack of which makes all other virtues useless and sometimes even harmful. Slowly the barbarous Greeks, the Hellenes, began to erect their own civilization on the ruins of the civilization they had wrecked."
The full citation is:
“The Dawn and Sunrise of History,” by Theodore Roosevelt. The Outlook (New York), vol. 115, February 14, 1917. (It also appeared as a short offprint/pamphlet under the same title.)