Lt. William Calley and some members of his platoon were tried and convicted of the rape, torture and massacre of Vietnamese civilians (the Mylai massacre). Some of this groups behavior was so evil that some American soldiers eventually got medals for threatening to open fire on Calley and his soldiers in order to rescue civilians.
Kerry's drastic charge is that the Mylai massacre (along with other atrocities) was not an aberration, but in fact standard operating procedure by all forces and that ranks from private up to general knew it, accepted it and participated in it.