From Federalist Paper #75 [emphasis mine]:
However proper or safe it may be in governments where the executive magistrate is an hereditary monarch, to commit to him the entire power of making treaties, it would be utterly unsafe and improper to intrust that power to an elective magistrate of four years’ duration. It has been remarked, upon another occasion, and the remark is unquestionably just, that an hereditary monarch, though often the oppressor of his people, has personally too much stake in the government to be in any material danger of being corrupted by foreign powers. But a man raised from the station of a private citizen to the rank of chief magistrate, possessed of a moderate or slender fortune, and looking forward to a period not very remote when he may probably be obliged to return to the station from which he was taken, might sometimes be under temptations to sacrifice his duty to his interest, which it would require superlative virtue to withstand. An avaricious man might be tempted to betray the interests of the state to the acquisition of wealth. An ambitious man might make his own aggrandizement, by the aid of a foreign power, the price of his treachery to his constituents. The history of human conduct does not warrant that exalted opinion of human virtue which would make it wise in a nation to commit interests of so delicate and momentous a kind, as those which concern its intercourse with the rest of the world, to the sole disposal of a magistrate created and circumstanced as would be a President of the United States.
To have intrusted the power of making treaties to the Senate alone, would have been to relinquish the benefits of the constitutional agency of the President in the conduct of foreign negotiations. It is true that the Senate would, in that case, have the option of employing him in this capacity, but they would also have the option of letting it alone, and pique or cabal might induce the latter rather than the former. Besides this, the ministerial servant of the Senate could not be expected to enjoy the confidence and respect of foreign powers in the same degree with the constitutional representatives of the nation, and, of course, would not be able to act with an equal degree of weight or efficacy…It must indeed be clear to a demonstration that the joint possession of the power in question, by the President and Senate, would afford a greater prospect of security, than the separate possession of it by either of them.
The highlighted sentence applies, of course, to Obama and his “legacy.”
But brilliant though he was, and relatively far-sighted as well, Hamilton thought it very likely that people of integrity would continue to fill the office of the presidency:
And whoever has maturely weighed the circumstances which must concur in the appointment of a President, will be satisfied that the office will always bid fair to be filled by men of such characters as to render their concurrence in the formation of treaties peculiarly desirable, as well on the score of wisdom, as on that of integrity.
And that’s mostly been true, to a greater or lesser extent, concerning presidents in terms of foreign policy. But Obama’s “treachery” in the Iran deal isn’t just for the purpose of his own personal “aggrandizement” (something Hamilton did envision as a possibility). It is for some other reason as well, about which many theories have been advanced. I offered my own some years ago, here, and I see no reason to change it now.
[NOTE: It is also to be remembered that at the time Hamilton and the Founders wrote, the plan was that the Senate was to be composed of people elected by the legislators of their respective states rather than elected in a popular vote. See this for the history of the 17th Amendment, which was adopted in 1913 and which made the switch to a directly elected Senate.]