be justified on the grounds that only Germany, Britain, France and Japan were strong enough during this period to consider challenging the United States or attempting to thwart it in significant ways: they are the potential rivals against whom it is significant to measure U.S. resources. The measures have to be somewhat different for petroleum. The relevant resources here appear to be U.S. imports vs. excess production capacity(since in 1956-1957 and 1967 the United States helped to maintain the existing regime by shipping oil to Europe from its own wells), and oil imports as a percentage of energy supply, giving a measure of relative U.S. and European dependence on imports.