The U.S. high command had been expecting an attack in 1972 but the size and ferocity of the assault caught the defenders off balance, because the attackers struck on three fronts simultaneously, with the bulk of the North Vietnamese army. This first attempt by the Democratic Republic of Vietnam (North Vietnam) to invade the south since the Tet Offensive of 1968, became characterized by conventional infantry–armor assaults backed by heavy artillery, with both sides fielding the latest in technological advances in weapons systems.