When Republican Senator Tom Cotton publicly revealed his open letter to Iran's leaders on Sunday, he was threatening more than just Ayatollah Ali Khamenei.

Cotton's unprecedented letter, signed by 46 other Republican Senators, was also a shot at the Obama administration, at President Obama's foreign policy, and indeed at the entire way that foreign policy is made in the United States.

Here are the basics.

What is Tom Cotton's letter to Iran?

On Sunday, Cotton revealed to Bloomberg View reporter Josh Rogin that he had organized an open letter to Iranian leaders about their ongoing negotiations with the US and other world powers over limiting Iran's nuclear program in exchange for relief from economic sanctions. Cotton, and the 46 other Republican Senators who joined in the letter, warned that even if Iran and the US did reach an agreement, that Congress or a future president could revoke or alter the agreement, especially if the US-Iran deal was not formally approved by Congress.

Cotton, and Senate Republicans generally, have been on record as opposing US-Iran nuclear talks. The implication of the letter was clear, then: a threat to blow up any US-Iran agreement if Republicans decide they do not like the terms.

Here is the text of the letter. Something you'll notice is that it takes an oddly patronizing tone, speaking to Iranian leaders in a manner that can seem deliberately condescending:

Senate Republican letter to Iran's leaders

Why is the letter a big deal?

This letter is very significant for three big reasons:

1) It's a deliberate effort to blow up the US-Iran nuclear talks. One of the biggest problems in the negotiations is trust: how to get the US and Iran to overcome decades of antagonism and to trust one another to follow through on their respective ends of the bargain. Republicans in Congress, who largely oppose the talks, have been trying to blow up negotiations by imposing new sanctions on Iran (something US negotiators promised not to do during talks), thus destroying trust between the two countries and forcing Iran to walk away.

This is the purpose of Cotton's letter: to convince Iran's leaders that Obama's promises could be undermined by Republicans, that the US cannot be trusted to uphold its end, and thus that Iran should quit the talks.

2) This is an unprecedented breach in protocol — and maybe even the law. Constitutional law sets sharp, and important, limits on Congress's involvement in foreign policy; unlike with legislation, the law defines the president as the "sole organ" in making foreign policy. Cotton's letter, by interfering with Obama's foreign policy, goes well beyond those limits and could very easily be seen as unconstitutional (much like Republicans' recent invitation to Israeli Prime Minister Benjamin Netanyahu to speak to Congress against Obama's Iran talks).

The letter also seems to potentially violate a law called the Logan Act, which forbids Americans from interfering in US foreign policy. The law was originally passed when a state legislator sent a letter to France, in 1798, undermining US policy toward France. Prosecution under the Logan Act isn't something that really happens, and Cotton will not be dragged before the Supreme Court. But this letter goes way beyond the legally articulated limits on Congress' role in foreign policy. It is not just undermining Obama's Iran talks, but the very foundations of how foreign policy is made in the United States.

Between Bibi invite and Cotton letter, GOP are blazing new trails in politicization of foreign policy--and debasement of their institutions. — David Rothkopf (@djrothkopf) March 9, 2015

3) This could dramatically alter the politics of the Iran deal in Washington. It's too soon to say precisely how the politics will play out here, but the implications could be far-reaching. By taking this extreme step, Cotton is almost certainly going to polarize the politics of the Iran deal and heighten the partisan nature of it. That could mean polarizing Republicans against the deal — he got dove-ish GOP Sen. Rand Paul to sign on, after all — but it could also mean polarizing Democrats in support of the deal.

Either of those outcomes would be significant and could change the outcome of Obama's years-long effort to negotiate a nuclear deal with Iran. If Obama wants to get his Iran deal ratified as a formal treaty, he will need some Republican support in the Senate. Cotton's letter could help make sure that doesn't happen. At the same time, if Obama decides to just make the US end of the deal an executive agreement (this is common for international agreements), then all Obama will need is for Congress to avoid passing new sanctions; in other words, to do nothing. Cotton's letter could help Obama in this case, by polarizing Democrats (some of whom had previously pushed for sanctions) such that they won't want to join Republicans in passing new sanctions.

What do Tom Cotton and the other Republican signatories want?

Cotton and a number of other Congressional Republicans are earnestly opposed to any deal whatsoever between the US and Iran and worry that Obama's negotiations will put America at risk. Their view is that Iran's leaders are so fundamentally aggressive and untrustworthy that they are almost certainly negotiating in bad faith and will exploit any deal to further their nuclear program.

Though many analysts find this argument unpersuasive, it is a valid position, and it's fair play to oppose the Iran deal on those grounds. But that opposition has grown to such an extent that Cotton has crossed a line from opposing an Iran deal to actively undermining not just the deal but US foreign policy itself.

It is also important to note that, while Cotton and other Republicans say that they do not oppose any nuclear deal with Iran but rather just the terms of this deal, that is difficult to believe for two reasons. First, negotiations are still ongoing and a deal still taking shape, so it's odd for Republicans to insist that the terms are so atrocious that they have no choice but to blow up negotiations, when the terms are still forthcoming.

Second, Cotton has actually been quite clear in the past that he does want talks to fail no matter the terms and believes that the only appropriate solution to Iran's nuclear program is outright regime change, which would almost certainly require war to achieve.

The underlying idea that Cotton had expressed is one that is common in more hawkish, neoconservative worldviews: that the core problem driving Iran's nuclear program is the fundamental nature of the regime, that this fundamental nature makes it impossible for the US to ever trust Iran and thus the US should never engage in negotiations, and finally that the only solution to this deeper problem is to remove that regime by force. This view is politically unpopular, though, as many Americans remember similar arguments leading up the disastrous 2003 Iraq invasion.

Is the letter working? Has it blown up Iran negotiations and/or rallied Congress against Obama?

It doesn't appear to have done either, but it will be difficult to tell for some time.

Shortly after the letter was published, Iranian Foreign Minister Javad Zarif published a response, the text of which was noticeably passive aggressive and mocking. Zarif stated that he saw Cotton's letter as a "propaganda ploy" meant to undermine Obama and suggested that Iran would disregard its threats. In other words, Zarif was saying that Iran saw through the letter and would not let it impact the ongoing negotiations.

But Zarif, a US-educated political moderate, does not represent the entirety of Iran's government. Iran's very unusual political system has an elected president (Hassan Rouhani, also a moderate who supports negotiations), but it also has an unelected supreme leader who is above the president. That supreme leader is Ayatollah Ali Khamenei, a hard-liner who has given the nuclear talks support but is also susceptible to pressure from more severe Iranian hard-liners who oppose talks. So it's possible that hard-liners, or even Khamenei himself, will not disregard the letter as Zarif did and that this could make Iran more hesitant to strike a deal.

Adding an extra degree of uncertainty to all this, on Tuesday morning AP reported that Zarif had issued a new statement, this one saying that Cotton's letter showed that the US was "not trustworthy." Had Zarif changed his mind? Was he trying to placate hard-liners? Had they pressured him to change tacks? It's not clear, and that speaks to the uncertainty of all this.

Meanwhile, in Washington, Cotton's letter has been extremely polarizing. So far, seven Republican senators have come out against the letter, saying it went too far or, in the case of Senate Foreign Relations Committee Chairman Bob Corker, warning that it could politically backfire and make passing new sanctions more difficult.

Vice President Joe Biden said in a statement, "In 36 years in the United States Senate, I cannot recall another instance in which senators wrote directly to advise another country — much less a longtime foreign adversary — that the president does not have the constitutional authority to reach a meaningful understanding with them."

Has a nything funny happened as part of this?

Oh yes. Much has been made of the tone of Cotton's letter, which comes across like a super-condescending Schoolhouse Rock rehash. For example, more than a few puzzled observers have pointed out that Cotton's letter explained term limits to an Iranian president who is himself term-limited.

Critics of the letter have also gleefully shared this blog post, by Harvard constitutional law scholar and former Bush-administration lawyer Jack Goldsmith, pointing out that Cotton's letter explaining the Constitution actually got part of the Constitution wrong. The error was pretty minor: Cotton said that the Senate ratifies treaties, when technically the Senate votes to "gives its advice and consent, empowering the president to proceed with ratification." Goldsmith conceded it was a "technical point," but it's never ideal to have a Bush administration legal heavyweight write, "in a letter purporting to teach a constitutional lesson, the error is embarrassing."

And the tone of Zarif's official response, an act of diplomatic trolling, is, depending on your point of view, either hugely insulting or hilarious (or maybe both). He wrote, for example, "it seems that the authors not only do not understand international law, but are not fully cognizant of the nuances of their own Constitution when it comes to presidential powers in the conduct of foreign policy."