Trading was an essential part of life, even before Norsemen became Vikings. Their ships could sail up rivers from the sea to the center of a country. They could land almost anywhere without a harbor or pier. Vikings became raiders out of necessity and then from the sheer desire for wealth. Early raids by men in several longships led to big fleets of ships containing armies of men who carried out full-scale invasions. In time Vikings built great trading centers where Norse traders met foreign traders from around the world.