Why National Security is an ambiguous concept or symbol?

I don't know but George Bush jr. explained the invasion to Iraq by the doctrine of National Security. He said except else that the war is a matter of the US coast guard and this is a nonsense. (According to him, the most important thing is to avoid a war on the US territory. This was the doctrine of Vietnam war, too.)

Because it's not like a chair or stove you can point to and say there it is.