The tactics involved in a defensive strategy have seldom been successful, but South Vietnam had no choice in the matter. Communist North Vietnam had the larger population and could field a larger army. North Vietnam also enjoyed unlimited war fighting material support from the Soviets and Chinese where the South, due to the U.S. Congress refusing to approve funding for the war, had ran out of ammunition by the time the North invaded in April 1975.
As the defender, South Vietnam was forced to spread its forces thinly in an attempt to defend the entire country where North Vietnam, in the offensive role, had the luxury of being able to mass their forces and over run each South Vietnamese outpost one at a time.
South Vietnam agreed to the 1973 Peace Accords that ended the war only because the U.S. assured them we would return if they were subsequently invaded by the North. When we departed, we assured our allies we would return if they were invaded, but as everyone now knows, we lied.
South Vietnam's President summed it up with these final words: "At the time of the peace agreement the United States agreed to replace equipment on a one-by-one basis. But the United States did not keep its word. Is an American's word reliable these days? The United States did not keep its promise to help us fight for freedom and it was in the same fight that the United States lost 50,000 of its young men."
With the election of a Democrat President and a Democrat Congress, our Iraqi and Afghan allies now have reason to be nervous; they know from historical precedent that Democrats are a scurrilous lot without any pretense of honor among them.
Notable among them was Senator Ted Kennedy, who blocked even medical supplies.
The years since have not been kind to the Kennedy clan.