If you were to draw a line across the map of Indochina, connecting Hanoi, the northern capital, to the nearby port city of Hai Phong, you would notice that about 40 miles south of that line's midpoint, there's a third city - a place called Nam Dinh. Early last July, while it's last neighbours to the north looked on helplessly. Nam Dinh became the first city in the Red River Delta to fall to the communists. By then it was too late for Hanoi or Hai Phong to do anything but resign themselves to the inevitable. But, for the rest of the free world, there is still time - although less than we may think - to profit from the bitter lesson learned by at least one of the people of Nam Dinh, during that city's last hours of freedom.