We see hopeful speculations pop up now and then on the possibility of some kind of left-right alliance against the various abuses associated with the national security state and unilateral Presidential warmaking.
The problem is that many of the so-called libertarians who occasionally voice seemingly principled objections to one or another of those abuses are actually authoritarian cranks. Like Congressman Ron "Papa Doc" Paul, in an interview he did with The New American. (Rep. Ron Paul on Libya 03/25/2011) Papa Doc is well regarded at The New American, a publication of the segregationist, conspiracy-mongering John Birch Society, who swear up and down they aren't anti-Semitic but really don't seem to like them thar Jews very much. Papa Doc complains of President Obama's course of action intervening in the Libya War:
The argument is that he's getting authority from the U.N. and treaties allow - that's an international law, it becomes the law of the land. But there are limitations. You cannot amend the Constitution by treaty. What if the UN decided that we shouldn't have a First Amendment? Would you say, oh, this is OK because the authority comes from the United Nations? That would be preposterous. [my emphasis]
Papa Doc there gives us a near-incoherent hash of Old Right isolationist hostility to the United Nations and international law, extreme nationalism, Constitutional illiteracy, and one valid (though buried) legal point.
What leaped out at me from this statement from a supposed strict-constructionist libertarian was the sentence I bolded, "You cannot amend the Constitution by treaty." Uh, yeah, Papa Doc, you can. Article 6 of the document you claim to hold so sacred states:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding. [my emphasis]
This clear statement that treaties are the supreme law of the US along with the Constitution does mean that a treaty can alter the Constitution. And the courts have in fact upheld that provision in that way. Papa Doc's fans would presumably comma-dance over the fact that the text does not explicitly call a treaty an Amendment. But in fact, Papa Doc and the Birchers and other hardcore rightwingers who like to call themselves "constitutionalists" or part of the "constitutional movement" have very quirky interpretations of the Constitution.
The United Nations has been one of the favorite bogeymen of the Old Right isolationists since it was first proposed. If you wanted to squeeze the remnants of an accurate claim out of Papa Doc's statement, it is true that the United Nations cannot require a member nation to go to war. That's because the UN Treaty specifies that compliance with United Nations actions must be in accord with the constitutional processes of each member state. So by adopting the UN Treaty, the Congress of the United States did not thereby alter the war powers provision of the Constitution.
I'm probably being too generous in crediting him with a valid point there, because he didn't actually make that point explicitly. But that legitimate objection in Papa Doc's comment is buried not only in the mire of Bircher-style UN demonization. Old Right isolationists reject international military alliances in general as undesirable limits on American sovereignty. And they condemn international law as completely illegitimate.
The answer to Papa Doc's question on the First Amendment is straightforward: yes, if an international treaty ratified by the Senate contained provision altering some aspect of the First Amendment, that change would become the supreme law of the land. So far as I'm aware, no treaty has ever had such an effect. But it recalls a couple of noteworthy points.
The treaty restoring Austrian sovereignty and independence in 1955 (Staatsvertrag), signed by the four Second World War occupying powers the US, the USSR, Britain and France, requires Austria to suppress any attempts to restore the Nazi Party. In accord with that treaty, Austria has on their books and enforces laws against Nazi-type activity, including Holocaust denial. The Holocaust denier David Irving did prison time there for violating that law.
This is a different standard of free speech than that of the US under the First Amendment. The treaty doesn't restrict American speech internally at all. But it does represent an instance in which the US recognized in a formal way that some kinds of advocacy - in this case Nazi Party advocacy - has been so thoroughly discredited historically that it should not be permitted: at least in Austria.
(Do I support that particular legal requirement for Austria? The short answer is no. But a one-word answer can't cover all the considerations involved, including how European democracies recognize political parties legally.)
Defenders of freedom of religious and of a nonsectarian government in the United States can also rightly point to the 1797 Treaty of Tripoli that ended the US conflict with the Barbary Pirates, negotiated under Secretary of State Thomas Jefferson, as relevant to those questions:
As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan nation, it is declared by the parties, that no pretext arising from religious opinions, shall ever produce an interruption of the harmony existing between the two countries. [my emphasis]
The bolded provision about how the US government "is not, in any sense, founded on the Christian religion" is a statement of position. But it is a statement in a treaty that is part of the supreme law of the United States. And one that comes in a treaty negotiated under the authority of one of the greatest advocates for freedom of speech and religion and ratified by a Congress during the first decade of government under the Constitution.