The Pentagon in its Quadrennial Review called climate change a threat to national security that “may spark or exacerbate future conflicts.” The assessment was part of the Pentagon’s official review of military risks, and labeled global warming “an accelerant of instability” and “weakening fragile governments”.
The Quadrennial Review is available at http://www.defense.gov/QDR/QDR%20as%20of%2026JAN10%200700.pdf. Section V contains the above quotes.