18 SEP--This Day in US Military History

1964 – South Vietnamese officials claim that two companies from the North Vietnamese army have invaded South Vietnam.

A battle resulted in Quang Tri Province, just south of the Demilitarized Zone, but the North Vietnamese forces were defeated with heavy casualties. Since North Vietnamese main force units had not been seen in South Vietnam before, U.S. military advisers questioned whether these were actually North Vietnamese troops, but in fact Hanoi had ordered its forces to begin infiltrating to the South.
This marked a major change in the tempo and scope of the war in South Vietnam and resulted in President Lyndon Johnson committing U.S. combat troops. North Vietnamese forces and U.S. troops clashed for the first time in November 1965, when units from the newly arrived 1st Cavalry Division engaged several North Vietnamese regiments in the Battle of the Ia Drang Valley in the Central Highlands.

https://thisdayinusmilhist.wordpress.com/2014/09/18/september-18-2/