What/where does the constitution give the president the diplomatic power?

Hamilton, although he had expressed substantially the same view in The Federalist regarding the power of reception, adopted a very different conception of it in defense of Washington's proclamation. Writing under the pseudonym, "Pacificus," he said: "The right of the executive to receive ambassadors and other public ministers, may serve to illustrate the relative duties of the executive and legislative departments. This right includes that of judging, in the case of a revolution of government in a foreign country, whether the new rulers are competent organs of the national will, and ought to be recognized, or not; which, where a treaty antecedently exists between the United States and such nation, involves the power of continuing or suspending its operation. For until the new government is acknowledged, the treaties between the nations, so far at least as regards public rights, are of course suspended. This power of determining virtually upon the operation of national treaties, as a consequence of the power to receive public ministers, is an important instance of the right of the executive, to decide upon the obligations of the country with regard to foreign nations. To apply it to the case of France, if there had been a treaty of alliance, offensive and defensive, between the United States and that country, the unqualified acknowledgment of the new government would have put the United States in a condition to become as an associate in the war with France, and would have laid the legislature under an obligation, if required, and there was otherwise no valid excuse, of exercising its power of declaring war. This serves as an example of the right of the executive, in certain cases, to determine the condition of the nation, though it may, in its consequences, affect the exercise of the power of the legislature to declare war. Nevertheless, the executive cannot thereby control the exercise of that power. The legislature is still free to perform its duties, according to its own sense of them; though the executive, in the exercise of its constitutional powers, may establish an antecedent state of things, which ought to weigh in the legislative decision. The division of the executive power in the Constitution, creates a concurrent authority in the cases to which it relates.