I won't go into the background as to why, but in 1954, French Indochina was divided into four countries: Laos, Cambodia, North Vietnam (Democratic Republic of Vietnam or DRV) and South Vietnam (Republic of Vietnam or RVN). Ho Chi Minh and his Merrie Men controlled DRV while a group of anti-Communists (many of them originally from what became DRV) ran RVN. There was supposed to be an election about uniting DRV and RVN and selecting the national leadership. Ho Chi Minh, the man who led the fight that forced the French out of Indochina, was most likely to be selected as Head Honcho of unified Vietnam. John Foster Dulles, the American Secretary of State and a strong anti-Communist, got the election put on indefinite hold. When it became obvious that DRV could not get control of a unified Vietnam politically, Uncle Ho and the boys decided to take over RVN militarily.Kweli said:the north vietnamese fought to protect their country against the us + partners.
When Country A invades Country B without provocation, Country A is committing aggression. You may not think that DRV and RVN were two separate countries, but they were. DRV attacked RVN without provocation. The North Vietnamese were aggressors in the Vietnam War.