Speaking of the Iran deal, it was President Obama's unilateralism that paved the way for President Trump's unilateral rejection. In other words, one pen cancelled the other.
Check out this wonderful article by Professor Goldsmith:
On the one hand, the president can avoid the need for approval from the Senate or Congress and make the international deal despite domestic opposition.Moral of the story: Work within the US Constitution if you want to create a lasting legacy.
On the other hand, a political commitment has no binding force under international or domestic law—and there is thus a danger that it will not be honored by a subsequent president.
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