What is the main theme in the nonfiction book, Alexander the Great?

Mercer perhaps best sums up Alexander the Great's theme in the closing pages, when he quotes from W. W. Tarn's biography of the same title: "For whatever else he [Alexander] was, he was one of the supreme fertilizing forces of history. He lifted the civilized world out of one groove and set it in another; he started a new epoch; nothing could again be as it had been."