World War I propaganda poster.
Imperial Germany fully expected Belgium to capitulate within hours of its invasion in 1914. When Belgium did not, Germany reacted as if betrayed. When the Belgians actually held out for two weeks, the Germans reacted with unbridled fury. Troops cut the breasts off nuns, burned libraries, and destroyed museums.
The rapaciousness of the German attack shocked the world. In the United States, propaganda portrayed the Allied Powers as the guardians of knowledge. But it also wanted to stop people from buying books, because wood, glue, and ink were needed for the war effort. So propaganda posters pushed the idea of borrowing your book from a public library.