Answer:
"Officially" US troops invaded (the quote "incursion") Cambodia on 01 May 1970. The Kent State shootings occurred on 04 May 1970 protesting that "incursion" (invasion).
Un-officially US troops and aerial bombing had been occurring since day one (probably even in 1955).
Special Ops men had been recording, studying, photographing, sketching, and doing alittle destructive work on NVA armor (tanks), facilities, cache supplies, depots, routes (Ho Chi Minh Trail), and general troop movements for years.
North Vietnam had at least three armor regiments (PT76 & T54 tanks), and one of those tank regiments was KNOWN to be stationed in Laos/Cambodia; they were being tracked (trailed/tailed) by SF/Special Ops men.