‘Help your Uncle Sam win the war’: Extraordinary collection of First World War propaganda posters intended to boost US morale comes up for sale He bought them in antique shops and took collection with him when he moved to England in the early 1980s The United States only joined the war, which had been raging in Europe for almost three years, in April 1917 A unique collection of propaganda posters which were designed to encourage Americans to support their troops in the First World War is going up for sale this week.