Addressing the United Nations on Tuesday, President Trump took a tough line on one of his predecessor's legacy initiatives: the Iran nuclear deal.

"The Iran deal was one of the worst and most one-sided transactions the United States has ever entered into," Trump said. "Frankly, that deal is an embarrassment to the United States and I don't think you've heard the last of it, believe me."

That decisive framing, "believe me," suggests Trump won't certify Iran's nuclear deal compliance come the next deadline on Oct.15. And that raises a question: Why has Trump embraced this aggressive stance after twice previously certifying the deal?

Well, for one, it's clear that Iran has infuriated Trump. Part of Trump's ire is rooted in his distinction between Iran's leaders' claims of Islamic democracy and their actions in government. As Trump put it, "The Iranian government masks a corrupt dictatorship behind the false guise of a democracy."

That said, Trump is also impatient over Iran's malevolent activities around the world. Now in office for 9 months, Trump will have been briefed extensively on the Iranian hardliners and their great efforts towards damaging U.S./allied interests. As I've explained, hardliner activities in this area are often hyper-aggressive and exceptionally brutal.

But in his frequent refrains of the deal being "bad" and an "embarrassment," Trump also seems to believe that Iran is receiving outsize financial rewards from the nuclear agreement. While the Obama administration turned a blind eye to illicit Iranian expenditures (as I noted following the deal's signing), Trump believes that a better Iran deal first requires the current deal's lapse.

While that's a risky strategy, there's little doubt that something must be done to batten down the deal's leaky hatches. After all, whether in ballistic missile development, covert support for North Korea, or restricting inspector access to certain facilities, Iran is acting far outside the spirit of that which motivated Obama to agree to the deal. Regardless, Iran won't change unless it comes to see access to deal-related sanctions relief as contingent on its adapted behavior.

If nothing else, Trump just gave us his strongest signal yet that the Iran deal is in trouble.