For a great timeline of the American involvements, see the PBS Vietnam Online site and their Battlefield timeline. The Vietnam Archive Operations Database is an excellent source for information about all operations. Some of the significant events include:
Operation Ranch Hand, 1962-1971 [to clear vegetation] (Wikipedia info)
Gulf of Tonkin, 1964 (Wikipedia info)
Tet Offensive, 1968 (Wikipedia info)
Surprised at Tet: U.S. Naval Forces in Vietnam, 1968
Battle of Khe Sanh, 1968 (Wikipedia info)
Hue Massacre, 1968 (Wikipedia info)
My Lai Massacre, 1968 (Wikipedia info)
Easter Offensive, 1972 (Wikipedia info)
Paris Peace Accords, 1973 (Wikipedia info)
Operation Starlite, 1965 [1st major battle] (Wikipedia info)
Operation Crimp, 1966 [largest operation] (Wikipedia info)
Battle of Ia Drang, 1965 (Wikipedia info)
Operation Rolling Thunder, 1965-1968 [bombing offensive] (Wikipedia info)
Operation Dewey Canyon, 1969 (Wikipedia info)
Operation Menu, 1969 [bombing in Cambodia] (Wikipedia info)
Operation Lam Son 719, 1971 (Wikipedia info)
Linebacker and Linebacker II, 1972 (Wikipedia info)
Fall of Saigon - 1975 (Wikipedia info)