So therefore that allows the President to go back on a trade agreement he personally signed in his last term? I'm not going to disagree Canada should do more ab

So therefore that allows the President to go back on a trade agreement he personally signed in his last term? I'm not going to disagree Canada should do more about reducing Fentanyl, nor that Canada can't control it with better policies. I am not clear on why this allows the United States to go back on agreements and allows the President to threaten with tariffs that seem to change weekly.