Meaning of mountains divide seas unite?

The phrase comes from a time when land transport was difficult and expensive compared to shipping costs via water. A good example is the coastal colonies of the American colonial period, who could communicate with each other via sailboats, but who could not extend their territory inland past the Appalachian mountains using horses and wagons. When, eventually, new settlements were founded in the trans-Appalachian region, they were separate states, and seemed at the time quite distant from the original colonial centers.