In answering media questions on the U.N. Arms Trade Treaty (ATT), I have found that hosts frequently state, as a matter of fact, that treaties require a two-thirds Senate majority, and if they don’t get it, they have no legal effect. Like all things, it’s not that simple. Here’s a short primer on when and how treaties can have legal effect.

In order of complexity:

Treaties 101: According to the Constitution, the President “shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur.” Today, this normally means 67 votes.

Treaties 201: But treaties fall into two categories: self-executing and non-self-executing. Self-executing treaties have immediate legal effect in the U.S. after Senate ratification. Non-self-executing treaties take legal effect at the international level upon entry into force for the U.S., following Senate consent and ratification. But they have limited domestic legal effect until Congress passes, and the President signs, implementing legislation. Thus, the House of Representatives has an important role to play in our treaty process. Treaties are normally presumed to be non-self-executing unless they are explicitly self-executing. The most significant case on this subject was Medellin v. Texas (2008), won by Texas Solicitor General Ted Cruz.

Treaties 301: But once the President signs a treaty, and even before the Senate considers it, the U.S. holds itself bound to “refrain from acts which would defeat the object and purpose of [the] treaty.” This obligation lasts as long as the treaty is signed but not ratified. The obligation derives in part from the 1969 Vienna Convention on the Law of Treaties. President Nixon signed the treaty in 1970, but it has not been ratified. Thus, the U.S.’s “object and purpose” obligation itself derives partly from an unratified treaty. But more broadly, the U.S. State Department “considers many of the provisions of the Vienna Convention on the Law of Treaties to constitute customary international law,” and therefore the U.S. will generally act in a manner consistent with its terms.

Treaties 401: What are acts that would “defeat the object and purpose” of an unratified treaty? The answer is largely in the eye of the beholder. The 1986 edition of the Restatement of the Law: The Foreign Relations Law of the United States says that “it is often unclear what actions would have such effect.” It goes on to note that, in the case of the second Strategic Arms Limitation Treaty, signed by President Jimmy Carter in 1979, testing a prohibited weapon might violate the “object and purpose” obligation, but failing to dismantle one might not. The “object and purpose” obligation is a back door to something that is in the neighborhood of, but not the same as, Senate ratification without the Senate (or the House) being involved at all.

So, in the context of the ATT, if this conference produces a treaty that is open for signature, President Obama may sign it immediately. The U.S. will then hold itself to be under a legal obligation not to defeat the ATT’s “object and purpose.” The interpretation of this phrase will rest with the State Department’s lawyers, perhaps in a way directed by subsequent legislation, whose decisions cannot be predicted and are not easily subject to legislative oversight.

In short, in the realm of treaties as well as domestic policy, the “administrative state” is growing at the expense of legislative government. It is high time for Congress in general—and the Senate in particular—to take Treaties 301 and 401 out of the course catalog by carefully defining the “object and purpose” requirement.