During World War I, the 43-year-old Merriam joined the US Army Signal Corps, was commissioned a captain, and served on the federal government's Aviation Examining Board for the Chicago region. He was also on the federal government's Committee on Public Information, an independent government agency created to influence US public opinion and encourage American participation in World War I. From April to September 1918, he was American High Commissioner for Public Information in Rome, Italy, where he developed propaganda designed to sway Italian public opinion. His mission was not only to encourage the Italian public to keep Italy in the war on the Allied side but also to undermine support for socialist and communist political parties. He may even have used Rockefeller Foundation money to help convince socialist leader Benito Mussolini to support the war. During his time in Rome, however, Merriam usurped the prerogatives of the US ambassador and embassy staff, and his repeated clashes caused him to be sent back to the United States after just six months in the post. Merriam claimed to be deeply shaken by his experiences in Italy, although he did not make clear in what way his views had changed. He also engaged in an extramarital affair while overseas, which led to marital problems.