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I believe that there is a very easy way for Trump to stop the war in Ukraine: he must initiate an all-US referendum with suggestions to prohibit big sports for transgenders, and establish that there are only two genders, men and women. If such a referendum is performed, Russians will experience a cognitive dissonance – they will realize that democracy leads to prohibiting gender diversity – and their worldviews will evolute, so they would stop supporting Putin and his war. This is so simple…
I believe you are simplifying the problem of Puting being the ruler of Russia and the actual factors which keep him in his position of power, though. Russian propaganda, as any other kind of propaganda, adapt to continue to keep people oppressed and picture the leader and the government as superior, not matter what the adversary does in the west.
Russian people will not experiment cognitive dissonance, because the Klemlin won't simply broadcast policies done by Trump which contradicts their discourse, they will continue to give a spotlight and cherry pic whatever Trump and the leaders of Europe do which goes along with the official discourse of the Kremlin.
Also, Putin is authoritarian he does not care whether the population experiments contradictions, he will stay in power by force of he believes it is necessary.