Which of these national disastors for America will go down in history as having the greatest impact? My ranking: 9/11, JFK Assn, Pearl Harbor. Reasons: 9/11 turned us into a paranoid nation. All out of porportion to the actual damage inflicted. 3000 casualties in no great shakes in historical terms. JFK Assn ended thera era of innocence. The 1950s ended that day. Pearl Harbor - USA would have entered World War 2 sooner or later anyway. Really didnt change anything.