The Romans can be called the great inventors of myths with the purpose of propaganda. Think of
Or Augustus: he reunited the Roman Empire; part of his power was due to huge efforts in propaganda, visible e.g. in the mass of coins showing his face, being sent all over the empire. He understood very well, that different cultures used different symbols - and he used them for his propaganda.
Politically the Roman army was an important factor. Propaganda in that case was used for the soldiers on the one hand, but on the other hand also for demonstrating the power of the army to the people, so they could trust in its strength. Even then security was an essential factor of politics. As long as the army functioned, the Roman Empire did as well (
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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.