Sometimes people make decisions based on the content of their dreams. Whether this is a rational action or not is beside the point of this thread. I simply want to know about the most significant actions ever taken because the person in question was inspired by the contents of such a dream.
Perhaps the most important example that I am aware of is Constantine’s vision before the Battle of the Milvian Bridge. It was a critical engagement between two men who claimed the title of Roman Emperor, and the night beforehand, Constantine believed that God told him to have his men mark their shields with Christian symbolism (most commonly accepted to be the Chi-Rho sign). They won the battle afterwards, so this is taken to be a crucial moment in the Christianization of Rome.
I ask this question because, unless one happens to believe that some dreams are literally supernatural in nature (which is beyond the scope of discussion for this thread), this is a topic that comes with much alternative history potential. I vaguely recall scenarios on this board where Constantine believes that his vision was sent by a different deity, leading him to throw his weight behind a religion other than Christianity.