“The ultimate guide to decision should be our estimate at the time of the nature and extent of the American interest. There may be good reasons to use our resources to resist a troublemaking power which commits aggression against a weak and friendly state if the subversion of that state would be a significant gain to the troublemaker or a significant loss to us. Even then, we should have a reasonably accurate and encouraging estimate of the chances of success before we act. We cannot afford to stake our world standing on a lost cause or on one with unduly high risks of failure.”