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propaganda is an instrument of the ideology, its a "symbolic violence" (cf.Bourdieu). a study of the habitus (representations and actions) of the agents in the field can help you to understand how propaganda affects them. so I think that you should not try to understand how does propaganda affect collective sentiment, but you should try to understand how believers create theirs beliefs, how and why some people become "victim" of propaganda (you can see the explanation of the magic by marcel mauss) ps:srry for my english...
Propaganda is not always negative. It's a method. We are using tons of propaganda in our daily lives to prove our points and get our works done. Propaganda is a collection of information set together in order to lead the audience to a certain conclusion. It can be a political propaganda, a social propaganda or an educational propaganda like my response to your question. The information can be true and can be used to reach positive conclusions. For example, Anonymous is using a propaganda to open people minds to fight against the corrupt politicians. Some (Most) Politicians also are using false or half-true information to lead people to negative (false) conclusions. About the collective sentiment and war, We have two different types of propaganda. One is presented by the Peace Activists. I watched a video on YouTube showing photos of Iran and saying Iranians are not our enemies. On the other hand, the politicians and the establishment is using another propaganda saying Iranians are Islamist terrorists, we need to attack them and help them grow up. Hope I was a bit helpful! :)
Propaganda works because it either instills fear or pride. What I find interesting is the rate of effectiveness this tool has on different societies as a whole. Thought to ponder: Were an entire society taught from birth to objectively review all incoming information, instead of accepting as truth, would propaganda have any effect at all? How would this tool have to evolve if the minds of the recipients were not/less susceptible?