Soon, that could be Obama's nuclear deal with Iran. (Andrew Harrer/Bloomberg)

This post has been updated now that the Obama administration has reached a deal with Iran.

Pretty soon, Congress could have an up-or-down vote on President Obama's nuclear deal with Iran.

But don't let your lawmaker fool you: It's not written in stone that Congress has the right to approve or disapprove of major international negotiations. It's not even a law.

The dirty little secret of U.S. international negotiations is this: Exactly how our government approves of most of this stuff is decided on a case-by-case basis.

Depending on what the president calls a deal with another country -- is it a treaty? an executive agreement? -- and the political movement du jour, a deal like the one with Iran could need a two-thirds majority vote in the Senate to be approved or could slide by without any vote at all.

What's happening right now is somewhere in the middle.

Confused? We were too. So we called up professor Charles Stevenson, an American foreign policy expert at the Johns Hopkins School of Advanced International Studies, for clarification.

Here's what you need to know to really understand what's going on with Congress and Iran -- and all other future negotiations.

It's all in the name

Let's go back to ninth-grade civics class for a minute.

A treaty is basically a formal agreement between two countries -- it could be to limit nuclear weapons, it could establish the United Nations, it could agree to international space law. The list goes on.

The Constitution says the Senate must approve any treaty the president wants to sign by a two-thirds majority vote. (As was alerted to us, it's a common misconception the Senate ratifies treaties: According to official Senate rules, it approves or disapproves of ratification of a treaty. But we digress.)

The point is, getting 67 senators to agree on complex international negotiations is difficult. So in the 1930s, President Franklin Delano Roosevelt figured out a workaround: He simply wouldn't call his international negotiations treaties. There, problem solved. His "executive agreements" could now unilaterally be approved by him and only him.

"When is a treaty not a treaty?" Stevenson said. "When it's not called a treaty." Otherwise: "There's no other difference."

Naturally, other presidents picked up on this politically convenient avenue. According to some statistics, executive agreements are now signed in the United States more than treaties -- by a ratio of 10-to-1. (Also: In the eyes of international law, there's no difference between the two.)

"That's the way the presidents have been since at least FDR," Stevenson said. "If they think they can get away with it, they'll do an executive agreement."

In the 1960s, Congress caught on to this sly move and passed a law requiring presidents to notify the legislative branch of all executive actions signed. But that didn't really stop presidents from making treaties and calling them by another name. It just meant they had to give Congress notice.

During his speech announcing the nuclear deal with Iran, President Obama said he welcomed robust discussion of the agreement but would ultimately veto legislation that "prevents the successful implementation of this deal." (AP)
Here's what happens when Congress wants to chime in

By 2015, the precedent allowing Obama to be the sole approver of most of his deals with other countries has become clear. But sometimes Congress wants a say on these deals -- especially when it politically suits them to do so.

In that case, lawmakers must convince the president to let them have an up or down vote on the final deal. That's what happened with the Iran deal, which is unpopular with Republicans who don't want the United States to lift sanctions on the rogue nation. (Remember when GOP Sen. Tom Cotton of Arkansas wrote a letter to Iran's leaders urging them not sign a deal with Obama?)

So Congress reached an agreement with Obama under which it can vote to void any deal that is reached with Iran. With the deal now wrapped up, Congress could vote very soon. Obama's pretty safe, though, because as was clear even when he agreed to allow Congress to vote, he can veto any "no" vote if it passes both chambers in the Republican-controlled Congress. At that point, Republicans would need to get many Democrats to join them to overturn the veto.

As you may have guessed, congressional oversight of international deals can blow with the political winds of the moment. Stevenson pointed to a 2008 agreement between the United States and Iraq on how American troops would be treated in Iraq as a good example.

At first, leaders in the Democratic-controlled Congress thought they should vote on something so important and were about to ask President George W. Bush to give them that vote. But then they realized the deal included a date on when American troops would come home, and by approving it, they'd have to put their members on the record for the politically difficult decision. So they backed off and let the president ink the deal by himself.

Stevenson said the reverse happened in 2001 when Bush was signing a nuclear non-proliferation deal with Russia. Russian leaders wanted him to have to publicly defend the deal -- and therefore Russia -- so they would only cooperate if Bush agreed to make it an actual treaty.

One more thing….

There's one more international agreement we should mention: A trade agreement. With Obama narrowly getting approval from Congress last month to negotiate two of these deals with Europe and Pacific Rim nations, that's also been in the news recently.

[The trade deal, explained for people who fall asleep hearing about trade deals]

Trade agreements have much clearer standards for approval. A president wanting to make one usually goes to Congress and gets the OK to negotiate the deal -- a mechanism known as Trade Promotion Authority -- with informal input from Congress. (This is what Congress has provided Obama -- after some fits and starts.) As part of the Trade Promotion Authority (TPA), Congress gets a final yea-or-nay vote on the deal.

Which in practice sounds a lot like the Iran deal. But it's not. It's a trade agreement. (And Stevenson thinks Obama should have gone to Congress a lot sooner to get authority to negotiate TPA. But that's a whole other post we'll write some other time.)

The big takeaway

OK, we hope we didn't confuse you too much. The basic thing we want you to take away from this is the difference between a treaty (which has to be approved by the Senate) and an executive agreement (a treaty-except-not-officially-called-that which doesn't have to be approved by the Senate but that sometimes Congress gets to vote on.)

And if you had to read that several times to get it, take heart that our country's method for approving international deals is so convoluted, it even confuses the experts.

"There are a lot of moving parts in this," Stevenson said, "and it's unclear to me which accelerator or brake is held in every case by the president, or the Congress or somebody else."

Foreign policy, ladies and gentlemen.