I think the major theme of Walter Miller's "A Canticle for Leibowitz" is summed up best by the Poet: "Non cogitamus, ergo nihil sumus," the inverse of Descartes' famous line, meaning, "We do not think, therefore we are not." This story is about the problems associated with not thinking things through thoroughly. It is explained that the rulers of what is probably today's world were given nuclear weapons as a test by God, and told not to use them lest their neighbors use their own weapons. But the rulers didn't use too much thought and decided to use their weapons, resulting in global destruction and fallout, practically making the Poet's "nihil sumus" true.
Dom Paulo recognized this quite clearly when he asked Thon Taddeo who would "govern the use of the power to control natural forces." Taddeo wanted to release the knowledge stored up by the monks for the good of mankind, whom he accused of saving it up "for the day when Man is good ... and wise," which will never be. Paulo, on the other hand, was afraid that the rulers would not have learned from the past and that history would necessarily then repeat itself with destruction anew.
As it turned out, Paulo was right. A few hundred years after his death nuclear war again broke out. In that troubled time Brother Joshua thought that "maybe destiny is always right now, right here." I think this view is one which the unthinking rulers did not comprehend. Instead of thinking about their actions and their future impact and realizing that they were forging the destiny of mankind, the rulers thought simply about the present situation, and even then they didn't think too clearly. As was so well said by the Poet, the people didn't think, and their lack of thought led to their destruction.