According to the author, the United States has one President, but two presidencies. In matters of domestic concerns congressional support is required, as well as the support of the President's constituents, the citizens.
Domestic policy is generally easier to change if done incrementally over a period of years.
In foreign policy and national defense issues the President has much more latitude. The United States administration was very closely monitoring the actions of Hitler in Europe, which was distracting it from domestic affairs. Or Truman's Taft-Hartley Act affecting domestic labor disputes. And he can usually garner needed support if he can define a viable policy. This works well in foreign policy matters where the American public is less aware of the details, but place their confidence in the President's ability to act in the best interests of the country.