President Donald Trump will not kill the Iran nuclear deal on Friday.

But when he declares that it has not been in US interests, he will consign the proudest legacy achievement of President Barack Obama's second term to a deeply uncertain future -- and could even set off a train of consequences that could eventually lead to its collapse. Should that be the case, Trump, or one of his successors in the Oval Office, may one day face the fateful choice that the deal was supposed to circumvent -- whether to use military force to stop the Islamic Republic racing toward the bomb.