If you can get past the political question issue (that the conduct of foreign affairs is, perhaps more so that any other governmental activity, firmly committed to the political branches of government and thus the judiciary should stay out) you could ask the courts. I would guess your answer would be the general welfare prong of the spending clause though. US v. Butler adopted the Hamiltonian view of the clause (that it confers a power to spend for public purposes that is not constrained to spending only on the designated legislative powers, but instead allows spending on anything that "provides for the general welfare"): "the power of Congress to authorize expenditure of public moneys for public purposes is not limited by the direct grants of legislative power found in the Constitution." The courts have been quite deferential to Congress's determination of what provides for the general welfare. The Foreign Assistance Act of 1961 contains congressional findings as to why foreign aid provides for the general welfare of the citizens of the US: So to answer your question? Look in Article I, Section 8, Clause 1.