Roosevelt angrily complained about the foreign policy of President Wilson, calling it "weak." This caused him to develop an intense dislike for Woodrow Wilson. When World War I began in 1914, Roosevelt strongly supported the Allies of World War I and demanded a harsher policy against Germany, especially regarding submarine warfare. In 1916, he campaigned energetically for Charles Evans Hughes and repeatedly denounced Irish-Americans and German-Americans who Roosevelt said were unpatriotic because they put the interest of Ireland and Germany ahead of America's by supporting neutrality. He insisted one had to be 100% American, not a "hyphenated American" who juggled multiple loyalties. When the U.S. entered the war in 1917, Roosevelt sought to raise a volunteer infantry division, but Wilson refused.
Roosevelt's attacks on Wilson helped the Republicans win control of Congress in the off-year elections of 1918. Roosevelt was popular enough to seriously contest the 1920 Republican nomination, but his health was broken by 1918, because of the lingering malaria.