First, you are incorrect, Trump never "tore up a deal." Trump pulled the U.S. out of the agreement, Iran and Germany and France and the U.K. and Russia and China remained in the Obama deal. Trump withdrew the U.S. because the Obama deal was working for Iran and their allies. Iran developed missiles that could hit Berlin and London and had highly enriched uranium one step away from an atomic bomb. That's when the U.S. intervened. Shipping has been disrupted numerous times in the past, here's a timeline of previous conflicts:
The Strait of Hormuz is a key location in tensions between Iran and the West, and the seizure of the Stena Impero is just the latest example
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As Trump said so many times, the reason behind the actions and the new deal is to make sure Iran never has a nuclear weapon, just the opposite of the Obama plan which gave Iran a clear path to a nuclear weapon.