Some basic effects of Europeans coming to North America are that the Native Americans caught new diseases which included smallpox, influenza, measles, and chicken pox. Explorers used superior fire power, like single shot muskets, to conquer natives.
Natural fortifications also helps defend a country. For example the Alps in Italy defends them very well. Rivers can also be a defensive position a country can fall back to. So no, fortifications and defenses arent the only thing. Nature is able to help too.
