In 1989, the Chinese Communist Party responded to a student demonstration in Tianenmen Square by massacring the students occupying Tiananmen Square. In the wake of that massacre, both houses of Congress unanimously passed sanctions against China. President George H. W. Bush vetoed the bill and during the override attempt, 37 Republicans in the Senate changed their positions to sustain the president's veto. Since then, candidate Bill Clinton promised that he would not renew China's Most Favored Nation status the next time the issue came. The next time China's MFN status came up for renewal, Clinton was president, and he did not oppose. Same thing the following time China's MFN status came up. After that, Clinton made China's MFN status permanent, eliminating any future automatic review of that status.
The fate of MFN status for China in the years after Tiananmen provides a model for what would likely happen with the Iran nuclear deal if Obama's veto is sustained. Among President Obama's constant refrains supporting his deal is the claim that if Iran violates the terms of the agreement, we can always "snap back" the sanctions. The problem is that "snapping back" the sanctions after the deal is put into place will create the same disaster scenario that Obama claims would result now if the deal is rejected. The only difference is that rejecting the deal now would leave an outside chance to renegotiate a better deal while "snapping back" the sanctions would open no such opportunity. The result is that any argument used today to support the president's deal could be used to oppose "snapping back" the sanctions under any conceivable violation by Iran. In other words, any violation by Iran would give a choice of "let them get away with it" or war. If those are the choices the future will present to us, "snap back" will be as much of a paper tiger as the renewal process of China's MFN status.