Last night, Donald Trump was right about one thing: the disaster that is Iran nuclear deal. But Trump’s inability to disassemble Hillary Clinton’s defense of the pact was another of his debate blunders. Had he done so the evening might have been less of a lost opportunity for him to put away a Democratic nominee who seemed like she might be on the ropes.

Though Clinton was not the driving force behind the effort to appease the Islamist regime — that dubious distinction belongs to her successor John Kerry — the former secretary of state has become an ardent defender of the deal. Her answer to Trump’s attack on it echoed the statements of President Obama about there being no alternative.

Rather than merely spouting about the agreement being the result of bad negotiating skills, Trump made a couple of important points. He was right to say that international sanctions had Iran on its knees before Obama and Kerry began the secret talks in 2013 that led to concession after concession on the part of the West. By the time Kerry finished negotiating, not only had Tehran managed to retain its facilities and most advanced hardware but had gotten the West to agree that any limits on their activities would end after 10-15 years. This bad bargain was also accompanied by the release of more than $100 billion in frozen assets and the collapse of the sanctions that had given the West the leverage it had just thrown away.

In response, Clinton claimed the only alternative was war. This is a lie. Had sanctions been not only continued but also strengthened to the point where Iran lost its ability to sell any of its oil anywhere, the chance of a deal that fulfilled Obama’s 2012 campaign promise to end Tehran’s nuclear program could have been salvaged and a despotic regime might have been fatally weakened instead of strengthened. At least that’s what Trump should have said. Instead he spent the next few minutes getting suckered into going down the rabbit hole of defending his lie about opposing the Iraq War before veering off into an argument about whether the U.S. Navy should be more aggressive in its patrolling in the Persian Gulf.

This failure is all the more frustrating because Iran’s recent provocative behavior illustrates the fallacies Clinton was defending. Its continued funding of international terrorism (made all the easier by Obama’s gifts and hostage payments), illegal missile tests, and continued threats against Israel illustrate that Obama’s belief that Iran wanted to “get right with the world” has been proven false.

Of course, Trump’s support for Russia’s re-entry into the Middle East and his incoherent stance on fighting ISIS (he wants to kick its ass but thinks other nations will do America’s dirty work for it while the U.S. pulls out of the region) explained why the catastrophe in Syria wasn’t raised by either candidate on Monday night. Trump would have to choose between his crush on Vladimir Putin and his desire to be tougher on Russia’s ally Iran, with whom it has waged a brutal war in Syria, and he won’t.

So even on the one foreign policy issue where Trump got it completely right, his own muddled and contradictory policy statements undercut the edge he ought to have over Clinton.