“In some cases, the materials at stake will be viewed as so essential to national survival or economic well-being that compromise is unthinkable. It is difficult, for example, to imagine that the United States will ever allow the Persian Gulf to fall under the control of a hostile power, or that Egypt will allow Sudan or Ethiopia to gain control over the flow of the Nile River. In such situations, national security considerations will always prevail over negotiated settlements that could be perceived as entailing the surrender of vital national interests.”