Joseph Jacobs,
The Story of Geographical Discovery…
“ The next two centuries are taken up in universal history by the magnificent struggle of the Greeks against the Persian Empire—the most decisive conflict in all history, for it determined whether Europe or Asia should conquer the world. Hitherto the course of conquest had been from east to west, and if Xerxes' invasion had been successful, there is little doubt that the westward tendency would have continued. But the larger the tract of country which an empire covers—especially when different tribes and nations are included in it—the weaker and less organised it becomes. ”
