In 1912, as a result of his work to bring nations together through arbitration and cooperation, Root received the Nobel Peace Prize.
At the outbreak of World War I, Root opposed President Woodrow Wilson's policy of neutrality. Root actively promoted the Preparedness Movement to get the United States ready for actual participation in the war. He was a leading advocate of American entry into the war on the side of the British and French, because he feared the militarism of Germany would be bad for the world and bad for the United States.
He did support Wilson once the United States entered the war.
In June 1916, Root was proposed for the Republican presidential nomination but declined, stating that he was too old to bear the burden of the Presidency. At the Republican National Convention, Root reached his peak strength of 103 votes on the first ballot. The Republican presidential nomination went to Charles Evans Hughes, who lost the election to Democrat Woodrow Wilson.
In June 1917, at age 72, he was sent to Russia by President Wilson as leader of the so-called Root Commission to arrange American co-operation with the new revolutionary government. The AFL's James Duncan, socialist Charles Edward Russell, general Hugh L. Scott, admiral James H. Glennon, New York banker Samuel R. Bertron, John Mott and Charles Richard Crane were members of Root's mission. They traveled from Vladivostok across Siberia in the Czar's former train. Root remained in Petrograd for close to a month, and was not much impressed by what he saw. The Russians, he said, "are sincerely, kindly, good people but confused and dazed." He summed up his attitude to the Provisional Government very trenchantly: "No fight, no loans."