Youngstown is the most significant treatment of the subject. But I think the OP is asking a different question, namely, where does it come from at all?
There are two related sources for this. The first is in the nature of a sovereign as set out in United States v. Curtiss-Wright Export Corp., 299 U.S. 304 (1936). Any goverment, as part of the nature of governments generally, have certain indisputable powers. This includes the power to make war and conduct foreign affiars – the ability to do this is part of how we define sovereignity. Therefore, the Untied States, as a sovereign, must have this power somewhere, regardless of whether it’s spelled out in the Constitution. And as the “sole organ of the federal government in the field of international relations,” these powers are vested in the President. Curtiss-Wright, 299 U.S. at 320.
The second basis for inherent presidential power is in the nature of executive power. The Founders weren’t writing on a clean slate – they were establishing a government based on the government of England but with imposed limits. For instance, in Art. I the Constitution invests the legislative power in the Congress and describes its operation and powers in great detail. In Art. III, the Constitution invests the judicial power in the Supreme Court and such other courts as Congress may establish but it defines the operation of the courts almost not at all. It wasn’t necessary, because the Founders knew what courts did. So Art. III didn’t need to detail the operation of the courts, it simply needed to establish and invest them. The contrast to Art. I is significant because Congress under the Constitution was a very different creature than Parliament, which has no such limits on its power.
Art. II, investing the Executive Power in a president, splits the difference. It’s by no means as detailed as Art. I, but it has somewhat more than Art. III. The argument in favor of innate presidential authority says that the Founders understood Executive power the same way they understood Judicial power. And by not limiting the presidential powers in Art. II as they did the Congress’s in Art. I, the Founders intended the president to inherit the Executive power of the Crown, to the extent consistent with other constitutional provisions. The same way they intended the Judiciary to have the same powers as England’s courts, but did not intend the textually limited Congress to have the same powers as Parliament. (With the exception that the president’s term of office is measured in years, not in lifetimes.)
Speaking for myself, I tend to buy these arguments. Certainly, it seems to me, some entity or other needs to have powers to conduct foreign affairs, even in the absence of direct Congressional authorization, becuase it’s crucial in emergencies. Moreover, the absence of a potent Executive is the deficiency in the Articles that the Constitution was meant to abate. And I agree that when the Founders wrote the Constitution, they knew what the words “legislative,” " executive," and “judicial” meant, and they used them to describe a panoply of rights and responsibilities that everyone understood. Of course, we fought a revolution to get rid of the English government, but that’s why the Constitution was able to establish institutions of power as modified from the English model. Ergo, where the Constitution doesn’t modify such institutions, they are what the Founders understood them to be from English legal history.
That said, in my opinion, President Bush’s contentions about the unitary Executive are unsupportable. Though the president has considerable inherent power in my view, this power evaporates in the face of contrary constitutional provisions. Therefore, he does (again IMO) not have the power to violate, inter alia, the Bill of Rights or the right to Trial by Jury in Art III, Sec. 2, as he claims he does.
I realize this last bit was far afield. However, I feel that it’s important for a full understanding of this question that people who read this response recognize that even amongst the many of us who recognize inherenet Executive authority, this president is still making extraordinary claims.
–Cliffy