With World War I an entire system of ideas how wars work collapsed. Suddenly it was no longer mostly soldiers who had to fight. War became an engagement of every day's life. Everybody got involved.
Propaganda therefore changed as well. The campaigns were no longer temporarily and recent but had to be planned for years. Who failed in organizing it or used the wrong keywords failed. Masters of modern propaganda became the British, whereas the Germans failed completely in the beginning.
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Munitions of the Mind. A history of propaganda from the ancient world to the present era. Manchester 1995 (2nd ed.)
This book gives a quite detailed insight on the tools and tasks of propaganda in European and /or Western history. Starting with ancient times the author goes up till the Gulf War and the meaning of propaganda today. In all those different eras propaganda was transporting similar messages, even when technical possibilities had not been fairly as widespread as today. Taylor's book is leading the reader through those different periods, trying to show the typical elements of each one.