cooler heads prevail
クリントン国務長官が緊張が高まる二国間関係をめぐり以前にも使っていた表現です。文字通りの意味なので解説は必要ないと思いますが、念押しでCambridge Dictionary of American Idiomsを引用しておきます。（UG）
cooler heads prevail
"Fig. the ideas or influence of less emotional people prevail. (Used of a tense situation.) "
One hopes that cooler heads will prevail and soon everything will calm down.
Clinton urges 'cooler heads' in China-Japan island dispute
UNITED NATIONS —
U.S. Secretary of State Hillary Clinton urged Chinese Foreign Minister Yang Jiechi Thursday to work to calm simmering tensions between China and Japan over disputed islands in the East China Sea.
“The secretary, as she has been urging for a number of months ... again urged that cooler heads prevail. That Japan and China engage in dialogue to calm the waters,” a senior State Department official said after the talks.
“We believe that Japan and China have the resources, have the restraint, have the ability to work on this directly and take tensions down. And that is our message to both sides.”
