In a side letter to the U.S.-Mexico agreement that is part of the NAFTA renegotiation, it appears that the U.S. has agreed that if it imposes tariffs on auto imports pursuant to Section 232, a certain amount of Mexican auto imports will be exempt. (You could also think of this as a cap on Mexican auto exports to the U.S. that will be tariff free.) The WSJ explained it this way last month:
Mexico and the U.S. have negotiated a side letter to the revamped North American Free Trade Agreement that would soften the blow of possible U.S. national-security tariffs the Trump administration is considering for auto imports, Mexican Economy Minister Ildefonso Guajardo said on Wednesday.
The side agreement comes as the administration earlier this year placed tariffs on steel and aluminum imports on national security grounds, citing section 232 of a 1962 trade law. It is now weighing whether to do the same with car imports, a move Mexico is taking pains to anticipate and which has implications for auto exporters Canada, the EU, Japan and South Korea.
Mexico has been concerned that even with an agreement on car imports under a new Nafta, its auto industry could still be vulnerable to tariffs of 25% the U.S. could impose under the so-called section 232 tariffs.
But with the side letter, if the U.S. applies those tariffs, Mexico would still have duty-free access to the U.S. market for cars that comply with new rules of origin—but only until certain limits on vehicle production are met, Mr. Guajardo said.
It's unclear to me why exactly Mexico views the side letter described here as useful. The only way I can make sense of it is that Mexico is taking the view that there are different categories of international agreements, with one more reliable than the other. We currently have the NAFTA and WTO tariff schedule obligations, which prohibit these tariffs (subject to the security exception), but Mexico seems to worry that the U.S. will not comply with those obligations. In addition, we may soon have this side letter, which also prohibits the tariffs for a certain quantity of imports from Mexico (again, subject to the security exception), but Mexico thinks the U.S. will comply with the obligations in the letter.
What's the difference between the NAFTA/WTO obligations and the side letter? Both are commitments by the U.S. government. However, only the side letter was signed by Trump himself (or his representative). That appears to give it some greater force and credibility in the eyes of Mexico.
Is there anything in international law or political science that distinguishes agreements between states from agreements between particular heads of state? Is the state not a unitary actor for all time, but rather the state can vary depending on who is in charge at a given moment?
The only alternative explanation I can think of is that in the side letter, the U.S. has specifically agreed that no national security exception will apply to this new obligation, which would make the two situations different. We'll have to see how exactly the side letter is worded to figure out if that's the case.