Many terrorist organizations are nationalists by another name.
Nationalist groups want freedom or independence from what they perceive as a outside power or force. Many terrorist organizations are formed to overthrow a government or gain access to nationalist state. ISIS for example wants an independent Islamic State which is free of Western influence. The Ku Klux Klan is a white nationalist group and also a domestic terrorist organization. Much of the terminology is perspective.
Romans were able to battle at Sea which helped them to expand their nation. Not all nations had ships and not all nations could fight on water and land. The romans had that advantage.