Wildavsky, Aaron. "The Two Presidencies," Trans-Action/Society, 4 (1966): 7-14.
“The United States has one president, but it has two presidencies; one presidency is for domestic affairs, and the other is concerned with defense and foreign policy. Since World War II, presidents have had greater success in controlling the nation’s defense and foreign policies than in dominating its domestic policies. Even Lyndon Johnson has seen his early record of victories in domestic legislation diminish as his concern with foreign affairs grows.
What powers does the president have to control defense and foreign policies and so completely overwhelm those who might wish to thwart him?
The president’s normal problem with domestic policy is to get congressional support for programs he prefers. In foreign affairs, in contrast, he can almost always get support for policies that he believes will protect the nation – but his problem is to find a viable policy.”
U Chicago Archives [pdf]