Tet 1968 Vietnam.

US. and South Vietnamese forces smashed the combat capability of the Viet Cong inflicting major damage on that organization for the first time. Casualties and degraded capabilities were so extensive that the North was forced to commit numerous regular army formations just to stabilize the situation. North Vietnamese leadership even briefly discussed ending the conflict among themselves.

Shortly thereafter, American public opinion concluded that we couldn't win, and agitation to cut and run mangified greatly. By getting their irregular forces smashed and negating their own ability to win, the North won.