We heard an interesting story from a friend of ours who was in the Special Forces during the Vietnam War. He said if you flew over the demilitarized zone between area controlled by the two opposing forces, you would see outlines of these stars everywhere on the ground. The Americans experimented with various configurations that would enable them to efficiently build and hold an outpost with only a few men, and this design turned out to be superior to all others.
Imagine walls in the shape of this symbol, and defenders on the walls. An approaching enemy from any direction would be caught in a crossfire. If it appeared that one of the walls would be breached, the commander would withdraw the defending forces inside the inner hexagram and seal it off. He would wait for the triangular point of the star to fill with enemy troops and then blow it up.
So even in recent years, this Star of David, or Shield of David, protected American forces in Vietnam. Amazing.